On 28/03/2022 19:38, Ulf Hansson wrote:
On Mon, 28 Mar 2022 at 12:11, Michael Wu <michael@xxxxxxxxxxxxxxxxx> wrote:
On 25/03/2022 18:13, Ulf Hansson wrote:
On Fri, 25 Mar 2022 at 06:46, Michael Wu <michael@xxxxxxxxxxxxxxxxx> wrote:
On 24/03/2022 19:27, Ulf Hansson wrote:
On Thu, 17 Mar 2022 at 10:14, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
On Wed, 16 Mar 2022 at 17:08, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
On 16.3.2022 16.46, Christian Löhle wrote:
So we are not going to let the block layer know about SD cache?
Or is it a separate change?
I have some code for this laying around, but as it requires reading, parsing and writing Function Registers,
in particular PEH, it's a lot of boilerplate code to get the functionality, but I'll clean it up and send a patch in the coming weeks.
We have the sd cache flush. We would presumably just need to call blk_queue_write_cache()
for the !mmc_card_mmc(card) case e.g.
if (mmc_has_reliable_write(card)) {
md->flags |= MMC_BLK_REL_WR;
enable_fua = true;
}
if (mmc_cache_enabled(card->host))
enable_cache = true;
blk_queue_write_cache(md->queue.queue, enable_cache, enable_fua);
To me, this seems like the most reasonable thing to do.
However, I have to admit that it's not clear to me, if there was a
good reason to why commit f4c5522b0a88 ("mmc: Reliable write
support.") also added support for REQ_FLUSH (write back cache) and why
not only REQ_FUA. I assumed this was wrong too, right?
Hi Ulf,
1. I've found the reason. If we only enable REQ_FUA, there won't be any
effect -- The block layer won't send any request with FUA flag to the
driver.
If we want REQ_FUA to take effect, we must enable REQ_FLUSH. But on the
contrary, REQ_FLUSH does not rely on REQ_FUA.
In the previous patch(commit f4c5522b0a88 ("mmc: Reliable write
support.")), REQ_FLUSH was added to make REQ_FUA effective. I've done
experiments to prove this.
Thanks for doing the research and for confirming.
Note that this is also pretty well documented in
Documentation/block/writeback_cache_control.rst.
Thanks for reminding. I'm clear now.
2. Why block layer requires REQ_FLUSH to make REQ_FUA effective? I did
not find the reason. Does anyone know about this? Thank you.
The REQ_FLUSH indicates that the storage device has a write back
cache, which also can be flushed in some device specific way.
The REQ_FUA (Force Unit Access), tells that the data can be written to
the storage device, in a way that when the I/O request is completed,
the data is fully written to the device (the data must not be left in
the write back cache). In other words, REQ_FUA doesn't make sense
unless REQ_FLUSH is supported too.
Thank you for your answer.
$subject patch should also conform to this pattern.
I'm not sure if I understood this in a right way... Did you mean I
should modify the subject of this mail/patch?
No, I just meant that the code in the patch should conform to this.
No problem. Please have a look at the code below.
If REQ_FUA is set, REQ_FLUSH must be set too.
However, it's still questionable to me whether we want to support
REQ_FUA through the eMMC reliable write command - in case we also have
support for managing the eMMC cache separately. It looks to me that
the reason why we currently support REQ_FUA, is because back in the
days when there was only the eMMC reliable write command available, it
was simply the best we could do. But it was never really a good fit.
I am starting to think that we may consider dropping REQ_FUA, if we
have the option to manage the eMMC cache separately - no matter
whether the eMMC reliable write command is supported or not. In this
case, REQ_FLUSH is sufficient and also a better match to what we
really can support.
Hi Ulf,
As to dropping REQ_FUA, I don't know if it is a good idea, but generally
we are facing three possible situations:
1. If both cache and reliable-write are available, both REQ_FUA and
REQ_FLUSH can be supported at the same time. In this case, with
available cache, the behavior of reliable-write is to write eMMC while
skipping cache, which is consistent with the current kernel's definition
of REQ_FUA. What's more, most eMMCs now support both cache and
reliable-write command.
Yes, this seems reasonable.
2. If only reliable-write is available, REQ_FUA should not be supported,
which is consistent with the current standard in another way. But I
don't think eMMCs that only support reliable-write can be easily found
nowadays.
If we drop REQ_FUA for this case, I am worried that we might break use
cases for those older eMMC devices.
So, no, let's keep REQ_FUA and REQ_FLUSH if reliable-write is supported.
OK. Let's keep them.
3. If only cache is available, we just use REQ_FLUSH. It is not in
conflict with keeping REQ_FUA.
Right.
Maybe, is it more reasonable to reserve FUA and use if/else to pick it
up or down, considering the compatibility? I mean, in most cases, FUA
and FLUSH are complementary. So it seems more feasible with branch to
choose.
Let's summarize what I think we should do then:
if (reliable-write supported) {
enable_fua = true;
enable_cache = true;
}
if (mmc_cache_enabled)
enable_cache = true;
blk_queue_write_cache(md->queue.queue, enable_cache, enable_fua);
Does this seem reasonable to you?
Yes. Let me attach the complete code here:
if (md->flags & MMC_BLK_CMD23 &&
((card->ext_csd.rel_param & EXT_CSD_WR_REL_PARAM_EN) ||
card->ext_csd.rel_sectors)) {
md->flags |= MMC_BLK_REL_WR;
enable_fua = true;
enable_cache = true;
}
if (mmc_cache_enabled(card->host))
enable_cache = true;
blk_queue_write_cache(md->queue.queue, enable_cache, enable_fua);
If this is good, I'll submit a patch-v2 soon.
[...]
Kind regards
Uffe
--
Best Regards,
Michael Wu
