Patch added to scsi-rc-fixes-2.6: [SCSI] arcmsr: fix message allocation

It would probably be easier if you could explain where you believe the memory IS used for DMA :)

Anyway, looking at ARCMSR_MESSAGE_READ_RQBUFFER
current code does this:

		ver_addr = kmalloc(1032, GFP_ATOMIC);

Here are the cases when that buffer is used:

checking for successful malloc: not DMA

		if (!ver_addr) {

copying the address: not DMA

		ptmpQbuffer = ver_addr;

memcpying 1 byte into the buffer: not DMA

			memcpy(ptmpQbuffer, pQbuffer, 1);

incrementing address: not DMA

			ptmpQbuffer++;

memcpying from the buffer: not DMA

		memcpy(pcmdmessagefld->messagedatabuffer, ver_addr, allxfer_len);

freeing the buffer: not DMA

		kfree(ver_addr);

Daniel

Sorry, maybe I did not ask distinctly enough.
I mean if I would like to allocate a memory space from ZONE_DMA for atomic context, why can I not use kmalloc(1032, GFP_ATOMIC|GFP_DMA)? In case of lack of GFP_DMA, kmalloc would grab the memory from ZONE_HIGH or ZONE_HIGHMEM, isn't it?(I read it from the textbook of Linux Kernel Development by Robert Love)
Or the basic is that you don't think it is necessary to allocate a memory space from DMA area?
Please give me some comments.
Thank you very much, :-）

-----Original Message-----
From: Daniel Drake [mailto:[email protected]]
Sent: Monday, February 25, 2008 6:15 PM
To: [email protected]
Cc: 'James Bottomley'; [email protected]
Subject: Re: Patch added to scsi-rc-fixes-2.6: [SCSI] arcmsr: fix message allocation

It would probably be easier if you could explain where you believe the memory IS used for DMA :)

Anyway, looking at ARCMSR_MESSAGE_READ_RQBUFFER
current code does this:

		ver_addr = kmalloc(1032, GFP_ATOMIC);

Here are the cases when that buffer is used:

checking for successful malloc: not DMA

		if (!ver_addr) {

copying the address: not DMA

		ptmpQbuffer = ver_addr;

memcpying 1 byte into the buffer: not DMA

			memcpy(ptmpQbuffer, pQbuffer, 1);

incrementing address: not DMA

			ptmpQbuffer++;

memcpying from the buffer: not DMA

		memcpy(pcmdmessagefld->messagedatabuffer, ver_addr,

allxfer_len);

freeing the buffer: not DMA

		kfree(ver_addr);

Daniel

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Um, no that's not true at all. GFP_DMA only allocates memory from ZONE_DMA and fails otherwise. You only need memory from ZONE_DMA if you cannot address physical memory above 24 bits (the old ISA restriction). This only applies if you're a standard ISA device and you're going actually to DMA to the memory in question. Neither of which applies in the arcmsr case, since you're only using the memory as a copy buffer within the kernel (it never sees an actual DMA transfer), and arcmsr doesn't have the ISA restrictions.

It's unnecessary because you never DMA to it, but even if you did, since arcmsr is a non-ISA device (with 64 bit DMA mask falling back to 32) you can just use ordinary kmalloc'd memory for that (provided you obey the coherence requirements).

James

James,
Appreciate for your answer.
One more question: do you think there are any compatibility issues on kmalloc/kfree pair?
I just trace the kernel code. It looks like it would not. But I am not quite sure absolutely.
May I have your comments?
Thank you,

-----Original Message-----
From: James Bottomley [mailto:[email protected]]
Sent: Monday, February 25, 2008 10:39 PM
To: [email protected]
Cc: 'Daniel Drake'; [email protected]; 'erich'
Subject: RE: Patch added to scsi-rc-fixes-2.6: [SCSI] arcmsr: fix messageallocation

or

Um, no that's not true at all. GFP_DMA only allocates memory from ZONE_DMA and fails otherwise. You only need memory from ZONE_DMA if you cannot address physical memory above 24 bits (the old ISA restriction). This only applies if you're a standard ISA device and you're going actually to DMA to the memory in question. Neither of which applies in the arcmsr case, since you're only using the memory as a copy buffer within the kernel (it never sees an actual DMA transfer), and arcmsr doesn't have the ISA restrictions.

It's unnecessary because you never DMA to it, but even if you did, since arcmsr is a non-ISA device (with 64 bit DMA mask falling back to 32) you can just use ordinary kmalloc'd memory for that (provided you obey the coherence requirements).

James

You mean does kmalloc/kfree obey the same rules as the coherent allocators in all regards except coherence ... pretty much, yes. There are some technical differences at the bottom in terms of page alignment, but nothing you need to worry about.

James