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

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

On Thu, Mar 31, 2016 at 8:14 AM, Lakshmi Sai Krishna Potthuri
<lakshmi.sai.krishna.potthuri@xxxxxxxxxx> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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