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  #1  
Old 14th October 2010, 11:59 AM
cadillackid Offline
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Custom Kernel HDA or SDA??

a stock fedora 11 install on my embedded board results in the primary CompactFlash card being driven by the SCSI driver and is called SDA.....

running my custom kernel which i have slimmed down and yanked out the SCSI stuff because I dont need it, results in my CompactFlash card being called HDA .. of course driven by the ATA driver...

which should I be using? both work just fine.. the CompactFlash card is on board and according to Soekris is actually run from an IDE controller, as there is no SCSI on board..

so im assuming the less amount of system overhead is to run it as IDE??

I have no issues running it either way I just want to do whats right.. and what is going to be supported in kernels down the line, as im considering a bump from my current 2.6.30 kernel upwards for my project...

-Christopher
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  #2  
Old 14th October 2010, 03:33 PM
DBelton Offline
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Re: Custom Kernel HDA or SDA??

hmmm.. for the past few years or so, the linux kernel uses libata for disk.. and all IDE/ATA, SATA, SCSI, USB, drives are assigned the sd* names, and CD, optical drives are named sr*

I guess when you removed the SCSI stuff, then it can't use the SCSI naming convention, so went back to using the IDE naming convention.

It is possible they are trying to move everything over to the SCSI naming convention and then eventually phase out the IDE naming convention, I don't know.

I do remember that before the change to libata, there were problems with support for some SATA drives, and also support for DMA, power management, and SMART. You may have reintroduced some of those problems when you took out the SCSI support on your system.
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  #3  
Old 14th October 2010, 04:19 PM
jpollard Offline
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Re: Custom Kernel HDA or SDA??

I believe that libata is still used - but to implement the IDE support.

The SCSI layer allows for a unified naming space for disks. All disks (IDE, SATA,
SAS,USB, and SCSI) can all be accessed generically. When the SCSI layer is
removed, it just exposes the underlying interface. In this case, IDE.

There should be no additional errors involved.
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Old 14th October 2010, 04:41 PM
DBelton Offline
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Re: Custom Kernel HDA or SDA??

sounds logical jpollard

Thinking about it, removing the SCSI layer shouldn't affect libata, and it was libata that corrected the problems. So you are probably correct, there shouldn't be additional errors.

Unless there are applications being written that specifically expect the sd& and sr* naming convention, and those should be able to be fixed with symlinks if necessary. (this would be poor coding if apps like this exist, but it's possible I guess)
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Old 14th October 2010, 04:55 PM
cadillackid Offline
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Re: Custom Kernel HDA or SDA??

I checked and libata is in fact what is running on my kernel.. the manufacture of the board also indicates that the CF socket is attached to the ATA / IDE bus.. so in my opinion running the SCSI layer could add some additional memory or resources needed..

I havent found any errors thus far in testing.. all works good...

applications are not an issue as im writing all the apps that run on this box and only for the hardware ive specified so im good there....

thank you for your help and quick response..
-Christopher
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Old 14th October 2010, 05:04 PM
DBelton Offline
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Re: Custom Kernel HDA or SDA??

only thing I can see that may be a problem.

Drives with 4k physical sectors, and those are going to become the norm. Possibly it may be supported by ATA before it becomes an issue for you..

(looks like this would only be an issue on the 4k sector drives that don't report a 512 logical sector size. most current drives still report 512 byte logical sectors)

From the linux site..

As of v2.6.33, Linux ATA drivers do not support drives with 4KiB logical sector size although there is a development branch containing experimental support[11]. For ATA drives connected via bridges to different buses - USB and IEEE 1394, as long as the bridges support 4KiB logical sector size correctly, the SCSI disk driver can handle them.

Last edited by DBelton; 14th October 2010 at 05:11 PM.
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  #7  
Old 14th October 2010, 07:00 PM
cadillackid Offline
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Re: Custom Kernel HDA or SDA??

I should be fine there since im using CF cards... the product is packaged with a 2 GB card, however I have run up to 16 GB CF cards in test on the ATA driver without issue...

however since the consensus seems to be after reading that everything is going to the SCSI naming convention, im going to benchmark CPU and RAM usage doing heavy "disk" reads and writes with each driver... if its negligible I'll go ahead and run the SCSI subsystem...

the interesting is when I watch the debug of the kernel Boot, the CF card always shows up as first an HDA drive.. then if the SCSI layer is loaded, it then shows up again as a SCSI device detected... and the "HDA" goes away and is replaced by "SDA".. it never shows as both once the kernel is fully loaded... it does take a bit of time to come online as I have to use a SCSI_WAIT_SCAN using the SCSI driver

taking the ATA layer out results in kernel build errors.. just pulling most of the ATA drivers results in a non-read of the initrd... putting initrd inside the kernel it gets a bit farther using RAMFS completely.. but will bomb trying to mount any partitions.. so that says to me the drive requires the ATA layer regardless of whether the SCSI system is loaded....
-Christopher
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  #8  
Old 15th October 2010, 06:43 AM
DBelton Offline
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Re: Custom Kernel HDA or SDA??

that would sound about right. From what I saw, libata really does the communication with the hardware, even when the SCSI layer is running.

It does seem kind of strange that they would pass everything through the SCSI layer, and rename it with the SCSI naming convention, though. I am thinking that decision was made about the time that SATA was coming out and they did that to correct some of the problems that introduced that SCSI was already handling properly.
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Old 15th October 2010, 01:12 PM
jpollard Offline
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Re: Custom Kernel HDA or SDA??

Quote:
Originally Posted by DBelton View Post
that would sound about right. From what I saw, libata really does the communication with the hardware, even when the SCSI layer is running.

It does seem kind of strange that they would pass everything through the SCSI layer, and rename it with the SCSI naming convention, though. I am thinking that decision was made about the time that SATA was coming out and they did that to correct some of the problems that introduced that SCSI was already handling properly.
It was also about the time of USB additions. Since ATA/SATA/PATA were
subsets of the SCSI protocol, it also made sense to combine these into
one naming area.

The SCSI layer became that common point because it already had the
naming, and the communication protocol that was common to all devices.
The merging allowed for the standardization of the activity. That
standardization simplified supporting structures - no longer needing
special handling for tapes, disks, scanners,... All were taken over by
the SCSI layer. This simplified interfaces to the block layer (anything that
worked like storage could be treated as a disk).
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