Re: [PATCH] libsas, aic94xx: fix dma mapping cockups with ATA

[Jeff Garzik <jeff@garzik.org>]

James Bottomley wrote:
> On Mon, 2007-07-16 at 16:19 -0400, Jeff Garzik wrote:
>> James Bottomley wrote:
>>> This one was noticed by Gilbert Wu of Adaptec:
>>>
>>> The libata core actually does the DMA mapping for you, so there has to
>>> be an exception in the device drivers that *don't* do dma mapping for
>>> ATA commands.  However, since we've already done this, libsas must now
>>> dma map any ATA commands that it wishes to issue ... and yes, this is a
>>> horrible mess.
>> Can you help me understand this logic?
>>
>> libsas must DMA map an ATA command... because libata also DMA maps an 
>> ATA command?  That does not make sense to me.
> 
> Why not? ... since the driver must *not* map an ATA command (to prevent
> double mapping from libata), it can't tell if the command comes from
> libsas or libata ... so libsas has to map any internal ATA commands it
> uses.
> 
> I think you're perhaps thinking libsas must map an ATA command libata
> has already mapped?  If so, no, libsas must only map the commands it is
> using internally independent of libata.

I was merely noting the patch description did not make sense to me, 
nothing more.  With this additional background, it does make sense :)

	Jeff