Re: GFP_DMA use in SCSI midlayer

[Mike Christie <michaelc@cs.wisc.edu>]

Mike Christie wrote:
> On Sun, 2006-10-01 at 15:40 -0500, James Bottomley wrote:
>> On Sun, 2006-10-01 at 22:13 +0200, Andi Kleen wrote:
>>> GFP_DMA in general is deprecated and should be replaced by appropiate
>>> dma_alloc_coherent()
>> Um, no it shouldn't.
>>
>> All of the places where we use GFP_DMA are because we might be
>> addressing an ISA card or other strange mask limited card (gated usually
>> by unchecked_isa_dma).  However, what is wanted in every case is
>> ordinary memory, not coherent memory.  They can't simply be replaced
>> with dma_alloc_coherent because
>>
>> a) it will waste memory for platforms that only do it in page size
>> multiples
>> b) It will fail on platforms that can't do it at all.
>>
>> Coherent memory is really only for device drivers to use in mailboxes
>> and shared ring buffers.
>>
>>> or similar.  And I can't imagine any modern systems still need them, and for 
>>> the few still non CONFIG_BROKEN ISA drivers maybe some other way can be found?
>>> And do they really require the mid layer data structures to be GFP_DMA too?
>> Well, it's the "or similar" that's still the problem.  All they need is
>> ordinary memory which respects the device mask (then I can get rid of
>> the unchecked_isa_dma flag as well).
>>
>> One could argue, I suppose, that when we get every SCSI path to go via
>> sg, then the block layer will bounce this for us, and we don't need to
>> worry at all.
> 
> 
> I think we might just need the blk_map_kern users now. For the async

Oops, I just meant for the ULDs, scsi-ml, and block layer drivers. I did
not look at if/when the LLDs inject commands directly to their queuecommand.

> execute I added the bounce code already and the block SG_IO has it
> atleady. I think the blk_map_kern bounce code got dropped because we
> thought the correct gfp_t would be passed in. But I think all we need is
> the patch below and all the paths are take care of. The patch is not
> tested. Patch was made against scsi-misc.
> 
> The last place that is sending non sg commands may just be md/dm-emc.c
> but that is is just waiting on alasdair to take some patches that fix
> that and a bunch of junk in there including adding bounce support. If
> the patch below is ok though and dm-emc finally gets converted then it
> will have sg and bonce buffer support.
> 
> 
> 
> Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
> 
> diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
> index 9c3a06b..e58b43a 100644
> --- a/block/ll_rw_blk.c
> +++ b/block/ll_rw_blk.c
> @@ -2533,6 +2533,7 @@ int blk_rq_map_kern(request_queue_t *q, 
>  	rq->bio = rq->biotail = bio;
>  	blk_rq_bio_prep(q, rq, bio);
>  
> +	blk_queue_bounce(q, &rq->bio);
>  	rq->buffer = rq->data = NULL;
>  	rq->data_len = len;
>  	return 0;
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> diff --git a/fs/bio.c b/fs/bio.c
> 
> 
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