RE: [PATCH V9 13/15] mmc: block: Add CQE and blk-mq support

[Hunter, Adrian]

> -----Original Message-----
> From: Bartlomiej Zolnierkiewicz [mailto:b.zolnierkie@xxxxxxxxxxx]
> Sent: Wednesday, October 4, 2017 12:40 PM
> To: Linus Walleij <linus.walleij@xxxxxxxxxx>
> Cc: Hunter, Adrian <adrian.hunter@xxxxxxxxx>; Ulf Hansson
> <ulf.hansson@xxxxxxxxxx>; linux-mmc <linux-mmc@xxxxxxxxxxxxxxx>; linux-
> block <linux-block@xxxxxxxxxxxxxxx>; linux-kernel <linux-
> kernel@xxxxxxxxxxxxxxx>; Bough Chen <haibo.chen@xxxxxxx>; Alex
> Lemberg <alex.lemberg@xxxxxxxxxxx>; Nowak, Mateusz
> <mateusz.nowak@xxxxxxxxx>; Yuliy Izrailov <Yuliy.Izrailov@xxxxxxxxxxx>;
> Jaehoon Chung <jh80.chung@xxxxxxxxxxx>; Dong Aisheng
> <dongas86@xxxxxxxxx>; Das Asutosh <asutoshd@xxxxxxxxxxxxxx>;
> Zhangfei Gao <zhangfei.gao@xxxxxxxxx>; Sahitya Tummala
> <stummala@xxxxxxxxxxxxxx>; Harjani Ritesh <riteshh@xxxxxxxxxxxxxx>;
> Venu Byravarasu <vbyravarasu@xxxxxxxxxx>; Shawn Lin <shawn.lin@rock-
> chips.com>; Christoph Hellwig <hch@xxxxxx>
> Subject: Re: [PATCH V9 13/15] mmc: block: Add CQE and blk-mq support
> 
> 
> Hi,
> 
> On Wednesday, October 04, 2017 09:39:45 AM Linus Walleij wrote:
> > On Fri, Sep 22, 2017 at 2:37 PM, Adrian Hunter <adrian.hunter@xxxxxxxxx>
> wrote:
> >
> > > Add CQE support to the block driver, including:
> > >     - optionally using DCMD for flush requests
> > >     - "manually" issuing discard requests
> > >     - issuing read / write requests to the CQE
> > >     - supporting block-layer timeouts
> > >     - handling recovery
> > >     - supporting re-tuning
> > >
> > > CQE offers 25% - 50% better random multi-threaded I/O.  There is a
> > > slight (e.g. 2%) drop in sequential read speed but no observable
> > > change to sequential write.
> > >
> > > CQE automatically sends the commands to complete requests.  However
> > > it only supports reads / writes and so-called "direct commands"
> > > (DCMD).  Furthermore DCMD is limited to one command at a time, but
> discards require 3 commands.
> > > That makes issuing discards through CQE very awkward, but some CQE's
> > > don't support DCMD anyway.  So for discards, the existing non-CQE
> > > approach is taken, where the mmc core code issues the 3 commands one
> at a time i.e.
> > > mmc_erase(). Where DCMD is used, is for issuing flushes.
> > >
> > > For host controllers without CQE support, blk-mq support is extended
> > > to synchronous reads/writes or, if the host supports
> > > CAP_WAIT_WHILE_BUSY, asynchonous reads/writes.  The advantage of
> > > asynchronous reads/writes is that it allows the preparation of the
> > > next request while the current request is in progress.
> > >
> > > Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> >
> > I am trying to wrap my head around this large patch. The size makes it
> > hard but I am doing my best.
> 
> I also think that this patch should be split on two patches. The 1st one
> introducing blk-mq and the 2nd one adding CQE support.
> 
> [ I don't agree that they make more sense together, on the contrary,
>   it is very difficult to properly analyze blk-mq changes on their
>   own while there are mixed with CQE related ones. ]

The CQE and non-CQE code paths are clearly marked.  But maybe you
are asking what the code would look like if we *never* had to support CQE.
And my point is we *do* have to support CQE. 

Do you have any questions about the code?