[RFC PATCH 2/2] spi: Add support for Zynq Ultrascale+ MPSoC GQSPI controller

[Harini Katakam]

Hi Mark,

On Thu, May 28, 2015 at 9:11 PM, Punnaiah Choudary Kalluri
<punnaiah.choudary.kalluri at xilinx.com> wrote:
> Hi Mark,
>
>> -----Original Message-----
>> From: Mark Brown [mailto:broonie at kernel.org]
>> Sent: Thursday, May 28, 2015 8:34 PM
>> To: Harini Katakam
>> Cc: Ranjit Abhimanyu Waghmode; Rob Herring; Pawel Moll; Mark Rutland;
>> ijc+devicetree at hellion.org.uk; Kumar Gala; Michal Simek; Soren Brinkmann;
>> devicetree at vger.kernel.org; linux-arm-kernel at lists.infradead.org; linux-
>> kernel at vger.kernel.org; linux-spi; Punnaiah Choudary Kalluri;
>> ran27jit at gmail.com
>> Subject: Re: [RFC PATCH 2/2] spi: Add support for Zynq Ultrascale+ MPSoC
>> GQSPI controller
>>
>> On Fri, May 22, 2015 at 08:43:54PM +0530, Harini Katakam wrote:
>> > On Fri, May 22, 2015 at 5:28 PM, Mark Brown <broonie at kernel.org> wrote:
>> > > On Wed, May 20, 2015 at 12:57:51PM +0530, Ranjit Waghmode wrote:
>>
>> > > Why is there a default case here?  That's going to men we try to handle
>> > > any random chip select that gets passed in as pointing to this lower
>> > > device which doesn't seem right.  The fact that this is trying to handle
>> > > mirroring of the chip select to two devices is also raising alarm bells
>> > > here...
>>
>> > This SPI controller has two CS lines and two data bus.
>> > Two devices can be connected to these and either the upper or the
>> > lower or both (Explained below) can be selected.
>>
>> > When two flash devices are used, one of the HW configurations in
>> > which they can be connected is called "parallel" mode where they
>>
>> I know what wiring chip selects in parallel is but that's not the
>> question - the question is about the handling of the default case.
>
> Yes, we should not handle default case here. We will change this.
>

Yes, that's right. I was just elaborating the mirroring part.

>>
>> > >> +static void zynqmp_qspi_chipselect(struct spi_device *qspi, bool
>> is_high)
>> > >> +{
>>
>> > >> +     if (is_high) {
>> > >> +             /* Manually start the generic FIFO command */
>> > >> +             zynqmp_gqspi_write(xqspi, GQSPI_CONFIG_OFST,
>> > >> +                             zynqmp_gqspi_read(xqspi, GQSPI_CONFIG_OFST) |
>> > >> +                             GQSPI_CFG_START_GEN_FIFO_MASK);
>>
>> > > No, this is broken - setting the chip select should set the chip select,
>> > > it shouldn't have any impact on transfers.  Transfers should be started
>> > > in the transfer operations.
>>
>> > This is the only way to assert the CS. It doesn't start transferring any data.
>>
>> OK, then you can't implement a separate set_cs() operation and shouldn't
>> be trying to do so.  This will break in multiple ways when the framework
>> tries to use the operations separately.  You probably need to implement
>> a single flat transfer() operation.
>
> As we said it will not start any data transfer. In order to set chip select (chip select only) we need to
> 1. Frame a control word with following parameters like the chip select that we would like to set and hold time
>  2. Update the control word to fifo
>
> To have a performance advantage (may be not trivial) we are executing this fifo entry along with the first
>  Data transfer entry in transfer function so that we can avoid waiting for fifo empty in set_cs function.
>
>  We will ensure CS assert by waiting till the fifo empty in set_cs function and justify the what the
> Function supposed do.
>

Yes, if we wait for cs assert to be executed (which is just the time
controller takes to fetch this command
and execute), this set_cs will be independent as expected. The
framework can use it anytime.

Regards,
Harini