Increasing MAXPHYS

jhell jhell at
Mon Mar 22 15:13:15 UTC 2010

On Mon, 22 Mar 2010 01:53, Alexander Motin wrote:
In Message-Id: <4BA705CB.9090705 at>

> jhell wrote:
>> On Sun, 21 Mar 2010 20:54, jhell@ wrote:
>>> I played with it on one re-compile of a kernel and for the sake of it
>>> DFLTPHYS=128 MAXPHYS=256 and found out that I could not cause a crash
>>> dump to be performed upon request (reboot -d) due to the boundary
>>> being hit for DMA which is 65536. Obviously this would have to be
>>> adjusted in ata-dma.c.
>>> I suppose that there would have to be a better way to get the real
>>> allowable boundary from the running system instead of setting it
>>> statically.
>>> Other then the above I do not see a reason why not... It is HEAD and
>>> this is the type of experimental stuff it was meant for.
>> I should have also said that I also repeated the above without setting
>> DFLTPHYS and setting MAXPHYS to 256.
> It was bad idea to increase DFLTPHYS. It is not intended to be increased.

I just wanted to see what I could break; when I increased DFLTPHYS it was 
just for that purpose. It booted and everything was running after. Wasn't 
long enough to do any damage.

> About DMA boundary, I do not very understand the problem. Yes, legacy
> ATA has DMA boundary of 64K, but there is no problem to submit S/G list
> of several segments. How long ago have you tried it, on which controller
> and which diagnostics do you have?

atapci0 at pci0:0:31:1:
class=0x01018a card=0x01271028 chip=0x24cb8086 rev=0x01 hdr=0x00
vendor     = 'Intel Corporation'
device     = '82801DB/DBL (ICH4/ICH4-L) UltraATA/100 EIDE Controller'
class      = mass storage
subclass   = ATA

I do not have any diagnostics but if any are requested I do have the 
kernel's that I have tuned to the above values readily available to run 

The first time I tuned MAXPHYS was roughly about 7 weeks ago. That was 
until I noticed I could not get a crash dump for a problem I was having a 
week later and had to revert back to its default setting of 128. The 
problem I had a week later was unrelated.

Two days ago when I saw this thread I recalled having modified MAXPHYS but 
could not remember the problem it caused so I re-enabled it again to 
reproduce the problem for sureness.

Anything else you need please address,




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