[PATCH 2/2] mtd: m25p80: consider max_transfer_size when reading

[Heiner Kallweit]

Am 05.04.2016 um 23:07 schrieb Brian Norris:
> On Tue, Apr 05, 2016 at 10:08:35PM +0200, Heiner Kallweit wrote:
>> Am 05.04.2016 um 21:39 schrieb Brian Norris:
>>> On Sat, Feb 27, 2016 at 11:50:11PM +0100, Heiner Kallweit wrote:
>>>> Some controllers have transfer size limits. To allow to deal with this
>>>> max_transfer_size was introduced in the SPI core recently.
>>>> Use this new feature to read in chunks if needed.
>>>>
>>>> Signed-off-by: Heiner Kallweit <hkallweit1 at gmail.com>
>>>
>>> Michal has been working on a similar series, with some differences (I'll
>>> comment below). I think his latest work is here:
>>>
>>> http://lists.infradead.org/pipermail/linux-mtd/2015-December/063865.html
>>>
>>> Is there a particular reason you've continued on your series instead of
>>> reviewing/fixing his?
>>>
>> Latest version of Michal's patch set was sent beginning of December and has
>> received no comment since then, so I assumed it's dead.
>> If that's not the case, totally fine with me and I'll have a further look at it.
> 
> There was a comment about the loop bounds changing away from page
> alignment. I think otherwise it's about ready to go.
> 
I had a closer look at Michal's patch set, few remarks:

Patch 2 changes the semantics of the return value of m25p80_read/write and the
related change to spi_nor_read/write is part of patch 4.
Means if the first patches are applied only we get a faulty behavior.
Usually this is undesirable, not sure whether it's acceptable here.

Patch 2
+	ret = m.actual_length - cmd_sz;
+	if (ret < 0)
+		return -EIO;
I think we should add special handling for the case ret == 0.
Usually this would indicate an error however there might be
intentional zero-length read's (not sure about that).
Therefore I'd propose to change the error condition to
if (ret < 0 || (!ret && len))
If zero-length reads are not possible we can simply change it to
if (ret <= 0)

Patch 7
W/o the proposed change to patch 2 the case that nor->read()
returns 0 isn't handled correctly.
We'd bail out from the read loop but return 0.
Instead we should return an error in this case.

With the change to patch 2 the case that nor->read() returns 0
can't happen and we should change the error condition to
if (ret < 0) for the sake of clarity.

Patch 8
Made it to mainline already, can be removed.

Patch 10
1. min_t isn't needed here because both arguments are of type size_t.
2. At least in the case of fsl-espi the size limit refers to one
   physical transfer (including the command) and therefore to the sum
   of all transfers.
We should change
+	t[1].len = min_t(size_t, len, spi_max_transfer_size(spi));
to
+	t[1].len = min(len, spi_max_transfer_size(spi) - t[0].len);

Apart from that the patch set looks good to me.

Heiner

> If you or Michal don't take a look at it, I'll rebase and resubmit it.
> 
> Brian
>