On Sun, 2 Dec 2018, Hannes Reinecke wrote:
> On 12/2/18 10:21 PM, Finn Thain wrote:
> > On Sun, 2 Dec 2018, Hannes Reinecke wrote:
> >
> > > Well, that lone 'kmap' is due to a quirk/errata in the datasheet;
> > > essentially
> > > we have to PIO a lone byte out of the FIFO to clear it up.
> > > And this byte is technically still part of the SCSI data, so we need to
> > > stuff it onto the end of the actual data sg list. Which is what the kmap()
> > > thingie does.
> > > So it really shouldn't be affected by the clustering algorithm.
> > >
> >
> > Sorry, I don't follow.
> >
> > If it's dead code, can it be removed
> >
> Oh, it's not dead code. It's required as per datasheet.
>
> > If it's not, does it require DISABLE_CLUSTERING?
> >
> No, not really. It just affects the very last byte of the sglist,
> so I can't really see how it should be affected by clustering.
>
AIUI, the scsi_kmap_atomic_sg() call which you added to esp_scsi.c assumes
that the sg list elements are page sized and page aligned.
DISABLE_CLUSTERING provides that guarantee, but am53c974.c doesn't use it.
Is this not a bug? What am I missing?
BTW, Documentation/block/biodoc.txt suggests a bounce buffer as an
alternative.
There are some situations when pages from high memory may need to be
kmapped, even if bounce buffers are not necessary. For example a
device may need to abort DMA operations and revert to PIO for the
transfer, in which case a virtual mapping of the page is required. For
SCSI it is also done in some scenarios where the low level driver
cannot be trusted to handle a single sg entry correctly. The driver is
expected to perform the kmaps as needed on such occasions as
appropriate. A driver could also use the blk_queue_bounce() routine on
its own to bounce highmem i/o to low memory for specific requests if
so desired.
--
> Cheers,
>
> Hannes
>
>
