[LINUX PATCH 1/2] mtd: Added dummy entry in the spi_transfer structure.

[Lakshmi Sai Krishna Potthuri]

Hi,

>-----Original Message-----
>From: geert.uytterhoeven at gmail.com
>[mailto:geert.uytterhoeven at gmail.com] On Behalf Of Geert Uytterhoeven
>Sent: Thursday, March 31, 2016 1:58 PM
>To: Lakshmi Sai Krishna Potthuri <lakshmis at xilinx.com>
>Cc: Mark Brown <broonie at kernel.org>; Michal Simek <michals at xilinx.com>;
>Soren Brinkmann <sorenb at xilinx.com>; David Woodhouse
><dwmw2 at infradead.org>; Brian Norris <computersforpeace at gmail.com>;
>Javier Martinez Canillas <javier at osg.samsung.com>; Boris Brezillon
><boris.brezillon at free-electrons.com>; Stephen Warren
><swarren at nvidia.com>; Geert Uytterhoeven <geert+renesas at glider.be>;
>Andrew F. Davis <afd at ti.com>; Marek Vasut <marex at denx.de>; Jagan Teki
><jteki at openedev.com>; Rafał Miłecki <zajec5 at gmail.com>; linux-
>mtd at lists.infradead.org; linux-kernel at vger.kernel.org; linux-
>spi at vger.kernel.org; linux-arm-kernel at lists.infradead.org; Harini Katakam
><harinik at xilinx.com>; Punnaiah Choudary Kalluri <punnaia at xilinx.com>;
>Anirudha Sarangi <anirudh at xilinx.com>; saikrishna12468 at gmail.com
>Subject: Re: [LINUX PATCH 1/2] mtd: Added dummy entry in the spi_transfer
>structure.
>
>On Thu, Mar 31, 2016 at 8:14 AM, Lakshmi Sai Krishna Potthuri
><lakshmi.sai.krishna.potthuri at xilinx.com> wrote:
>>>> >This is really not what I'd expect to happen, I'd expect that these
>dummy
>>>> >cycles would be in addition to the actual data (see my request for better
>>>> >documentation...).  If they overlap with the data then what is the point
>in
>>>> >specifying this?  It's more work for the host, what benefit do we get
>from
>>>> >doing it over just handing it like a normal byte?
>>>
>>>> len field in the transfer structure contains dummy bytes along with actual
>>>data
>>>> bytes, controllers which requires dummy bytes use len field and simply
>>>Ignore
>>>> the dummy field (contains only no of cycles)added in this patch.
>Controllers
>>>> (like ZynqMP GQSPI) expects dummy in cycles won't work directly by
>using
>>>> len field because host driver doesn't know that len field of a particular
>>>transfer
>>>> includes dummy bytes or not (and also number of dummy bytes included
>in
>>>len
>>>> field). In such cases driver use this dummy field to identify the number of
>>>dummy
>>>> cycles and based on that it will send the required number of dummy
>cycles
>>>(which
>>>> i did in the second patch).
>>>
>>>This doesn't make any sense at all to me.  Why does the controller care
>>>what the bytes being sent to and from the device mean?
>>
>> From the flash point of view, it expects the controller to send the dummy
>> on 1/2/4 lines based on the command. For Quad commands, flash expects
>> 4 lines to be active during dummy phase. Similarly, 2 lines for dual
>> Commands and 1 line for normal/fast commands.
>> Since len field contains total number of cmd+addr+dummy bytes,
>> host driver should take care of sending these bytes on their respective
>> bus widths. Knowing when the dummy is being sent also helps in
>> the correct switching of IO pads (since the data lines are bidirectional)
>> ZynqMP GQSPI is a generic controller, majorly interfaced to flash devices.
>> It seems reasonable for it to know the above information from
>> the flash layer. Adding "dummy" cycles entry should be useful to any
>> controller that cares about it without affecting other spi/qspi controllers.
>
>The m25p80 driver already uses dummy cycles, using real spi_transfer
>structs, which have tx_nbits/rx_nbits fields to indicate how many data lines
>to use.

m25p80 implementation just send command, address and dummy together
with tx_nbit field as 1 and host driver can't differentiate between them.
Command and address go on one line and dummy should send based
on the command as explained my previous mail.

Regards
Sai Krishna


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