[RFC PATCH 1/5] spi: introduce flag for memory mapped read

[Geert Uytterhoeven]

On Thu, Aug 6, 2015 at 3:51 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Thu, Aug 06, 2015 at 05:55:23PM +0530, Vignesh R wrote:
>> On the whole following are my requirements:
>> 1. to be able to communicate with non -flash SPI devices via config port
>> ( this functionality is supported by current driver, I dont want to
>> break it). Or pump any spi_message on to SPI bus directly.
>> 2. take advantage of memory mapped port in order to increase read
>> throughput( and use dma in future) when the slave is a m25p80 type flash.
>> 3. handle m25p80 as well as other slave on multiple chipselects.
>>
>> I just need to know whether the user that requested the transfer is
>> m25p80 driver. If yes, ti-qspi driver can take advantage of memory
>> mapped interface, else just use config port to access SPI bus directly.
>
> The problem with this approach is that it's an abomination.  It's adding
> a SPI-user specific hack which is detected by a specific driver.  That's
> really not sane - what happens when we have lots of these kinds of "I'm
> an X SPI-user" with drivers detecting that?  It's not maintainable in the
> long term.
>
> Yes, your requirements _today_ seem simple and easy, but you're only
> thinking about today, not tomorrow when you've moved on and someone else
> has to maintain the mess left behind (or delete it from mainline because
> they're sick of dealing with a hack.)
>
>> The spi_message that is received in transfer_one_message() is too
>> generic to imply the slave device that is on the other side of the wire.
>> IMO, the read command does not imply that the slave is m25p80 flash
>> (besides the read opcodes vary across vendors of m25p80 and across modes).
>
> I can see both sides of the argument.
>
> Mark is saying: if the SPI driver detects that the message to be transmitted
> is a read command followed by the appropriate number of dummy bytes, and
> then the data being read _and_ it's using quad-mode access, and the hardware
> generates _exactly_ that in hardware using the memory mapped mode, there is
> no reason _not_ to use the hardware to achieve that SPI transaction.  The
> bus activity will be identical to what happens when the SPI controller is
> used manually to achieve that bus sequence.
>
> You're saying: but the documentation says you can't use it for anything
> except m25p80.  If you look at 24.5.4.1.2, it tells you what the SFI
> generates on the bus, which is:
>
> 1. CS active
> 2. Read command byte sent
> 3. 1-4 address bytes sent
> 4. 0-3 dummy bytes sent
> 5. data bytes read from bus
> 6. CS inactive
>
> So, Mark's point is "if we can detect a transaction which fits _that_
> bus activity, there's no reason not to use this acceleration for the
> transaction."
>
> What you're failing to counter with is: we don't have enough information
> in the SPI driver to know how many dummy bytes there are between the
> address bytes and the data read from the bus.

Irrespective of the dummy bytes.
What if the spi device is not a FLASH ROM, but some other device,
which receives a data packet that accidentally looks like an m25p80 READ
command?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

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