RE: [PATCH v3 2/2] ufs: ufs-exynos: set dma_alignment to 4095

["Kiwoong Kim" <kwmad.kim@samsung.com>]

> On 2020-12-22 11:34, Can Guo wrote:
> > On 2020-12-22 11:17, Kiwoong Kim wrote:
> >>> On 2020-12-22 10:21, Kiwoong Kim wrote:
> >>> > Exynos requires one scatterlist entry for smaller than page size,
> i.e.
> >>> > 4KB. For the cases of dispatching commands with more than one
> >>> > scatterlist entry and under 4KB size, Exynos behaves as follows:
> >>> >
> >>> > Given that a command to read something from device is dispatched
> >>> > with two scatterlist entries that are named AAA and BBB. After
> >>> > dispatching, host builds two PRDT entries and during transmission,
> >>> > device sends just one DATA IN because device doesn't care on host
> dma.
> >>>
> >>> If my understanding is correct, above is same to all hosts, only
> >>> below part is Exynos's behavior. Please correct me if I am wrong.
> >> You're correct.
> >>
> >>>
> >>> > The host then tranfers
> >>> > the whole data from start address of the area named AAA.
> >>> > In consequebnce, the area that follows AAA would be corrupted.
> >>>
> >>> In consequence
> >>>
> >>> >
> >>> >     |<------------->|
> >>> >     +-------+------------         +-------+
> >>> >     +  AAA  + (corrupted)   ...   +  BBB  +
> >>> >     +-------+------------         +-------+
> >>> >
> >>>
> >>> AFAIK, queue->dma_alignment is only used in the case of direct-io,
> >>> i.e. in
> >>> blk_rq_map_user/kern(), which are mainly used in IOCTL.
> >>> If a request's buffer len and/or buffer start addr is not aligned
> >>> with
> >>> queue->dma_alignment, bio.c will make a bounce bio such that the
> >>> request
> >>> get a new buffer which starts on a new page. After the bounce bio is
> >> ended,
> >>> the data in the bound bio will be copied to the initial buffer.
> >>>
> >>> So in this fix, you are making sure the AAA and BBB are all mapped to
> >>> one
> >>> bounce bio and stay in one bi_vec, so when we do map_sg they come in
> >>> one
> >>> sglist, please correct me if I am wrong.
> >>>
> >>> If my understanding is correct, what is the real use case here -
> >>> why/how
> >>> user starts a request which can generate more than one sglists whose
> >>> sizes
> >>> are all under 4KB? I am just curious.
> >>>
> >>> Thanks,
> >>>
> >>> Can Guo.
> >>
> >> You nearly exactly got what I’m thinking.
> >> And I think there could be various cases making those situations,
> >> which are definitely up to user programs. That is the case using
> >> different memory areas to contain something.
> >>
> >
> 
> Hi Kiwoong,
> 
> Sorry if I made you confused. I think I know your intention here now.
> When bio.c makes the bounce bio, it allocates one new page and
> add this page to the bounce bio. In this way, only one bi_vec is
> needed and only one sglist entry shall be generated. Am I right?
> 
> Thanks,
> 
> Can Guo. 

Yes, that's it.


Thanks.
Kiwoong Kim
> >
> >> Thanks.
> >> Kiwoong Kim
> >>>
> >>> > Signed-off-by: Kiwoong Kim <kwmad.kim@samsung.com>
> >>> > ---
> >>> >  drivers/scsi/ufs/ufs-exynos.c | 9 +++++++++
> >>> >  1 file changed, 9 insertions(+)
> >>> >
> >>> > diff --git a/drivers/scsi/ufs/ufs-exynos.c
> >>> > b/drivers/scsi/ufs/ufs-exynos.c index a8770ff..8635d9d 100644
> >>> > --- a/drivers/scsi/ufs/ufs-exynos.c
> >>> > +++ b/drivers/scsi/ufs/ufs-exynos.c
> >>> > @@ -14,6 +14,7 @@
> >>> >  #include <linux/of_address.h>
> >>> >  #include <linux/phy/phy.h>
> >>> >  #include <linux/platform_device.h>
> >>> > +#include <linux/blkdev.h>
> >>> >
> >>> >  #include "ufshcd.h"
> >>> >  #include "ufshcd-pltfrm.h"
> >>> > @@ -1193,6 +1194,13 @@ static int exynos_ufs_resume(struct ufs_hba
> >>> > *hba, enum ufs_pm_op pm_op)
> >>> >  	return 0;
> >>> >  }
> >>> >
> >>> > +static void exynos_ufs_slave_configure(struct scsi_device *sdev) {
> >>> > +	struct request_queue *q = sdev->request_queue;
> >>> > +
> >>> > +	blk_queue_update_dma_alignment(q, PAGE_SIZE - 1); }
> >>> > +
> >>> >  static struct ufs_hba_variant_ops ufs_hba_exynos_ops = {
> >>> >  	.name				= "exynos_ufs",
> >>> >  	.init				= exynos_ufs_init,
> >>> > @@ -1204,6 +1212,7 @@ static struct ufs_hba_variant_ops
> >>> > ufs_hba_exynos_ops = {
> >>> >  	.hibern8_notify			= exynos_ufs_hibern8_notify,
> >>> >  	.suspend			= exynos_ufs_suspend,
> >>> >  	.resume				= exynos_ufs_resume,
> >>> > +	.slave_configure		= exynos_ufs_slave_configure,
> >>> >  };
> >>> >
> >>> >  static int exynos_ufs_probe(struct platform_device *pdev)