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

[Can Guo <cang@codeaurora.org>]

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.
> 
>> 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)