RE: support DUAL and QUAD[patch v1]

[<thomas.langer-th3ZBGNqt+7QT0dZR+AlfA@public.gmane.org>]

Hello Pekon,

Gupta, Pekon wrote on 2013-07-16:
> >
> > Hello Pekon,
> >
> > Gupta, Pekon wrote on 2013-07-16:
> > > >
> > > > Hi, Gupta
> > > >
> > > >
> > > > > [Pekon]: Instead of adding new fields you can use existing 'mode'
> field
> > to
> > > > > pass on the platform specific configurations. And if 'u8 mode' does
> not
> > > > > suffice you can increase it to 'u32'.
> > > > > #define QSPI_MODE       1 << 5; // just check which bit-fields are un-
> > used
> > > > > spi_board_info->mode |= QSPI_MODE;
> > > > >
> > > >
> > > > well, can dual and quad be regarded as a spi mode? if so, your comment
> > > > seems
> > > > to be right.
> > >
> > > Yes, Quad and Dual modes should be regarded as extension of SPI
> > protocol.
> > > - They follow the same basic principle of synchronous data transfer. Right
> ?
> > > - These modes are not adding any extra side-band | In-band signaling or
> > > controls to modify the protocol. They are just increasing the width of
> > >  data-channel for better throughput.
> > >
> > > with regards, pekon
> > >
> >
> > In general, yes. But I think, for the interface we have to take care of more
> > details.
> >
> > For example, what happens in the following situation:
> > We have a spi-controller, which supports the QSPI mode, and a spi-flash,
> > which fulfils
> > the requirements, but the board has not connected all signals?
> >
> [Pekon]: So, actual implementation should set QSPI_MODE based on both
> (1) device capabilities (detected in probe)
> (2) board-DT properties (this would take care of your other constrains)
> 
yes, and in the end the slave (as of m25p80) can decide, which actions/modes are usable
and should only use them.

> > And the interface for the slave-driver (like m25p80) should allow to specify
> > the transfer mode
> > for each spi_message.
> > This will be necessary, because it depends on the flash and mode, how
> each
> > phase of "cmd",
> > "address", and "data" will be transferred.
> >
> [Pekon]: Are you saying that same controller can send interleaved
> Quad-SPI and Single-SPI spi_messages to same device (here flash device)?
Yes, for example the S25FL129P from Spansion always expects the command (first 8 bit)
on a single wire and then, depending on the command, switching to dual or quad-mode
for the address and/or data phase.
And other flashes I have seen (don't remember name or manufacturer) support to be switched
into a quad-mode, in which they expect all data (including the command) as quad transfer.
> 
> Is it due to flash-device constrain that it accepts some flash-commands
> (like erase, etc) only on Single-wire-SPI, whereas  data transfers can be
> Performed on Quad-SPI mode ?
> In such case you need to add 'mode' field in struct spi_message, so that
> this information moves along with the spi_message in queue. And then
> when that message reaches spi_pump_message() and
> transfer_one_message() is called, then you can do needful configs.
Yes, I think this would be a good way to allow the flash driver to control these details.

> 
> However, I still don't think you need to control 'QSPI_MODE' at granularity
> of spi_transfer level, because m25p80 flash-driver use spi_write() API to
> pass data to SPI driver, which internally creates a spi_message in each call.
I expect some changes to this driver for adding this modes, but as discussed above,
it should be enough to have the mode per message.

> 
> Following best explains use of spi_message in /include/linux/spi.h
> ---------------------------------------------------
> * A @spi_message is used to execute an atomic sequence of data transfers,
>  * each represented by a struct spi_transfer.  The sequence is "atomic"
>  * in the sense that no other spi_message may use that SPI bus until that
>  * sequence completes.  On some systems, many such sequences can
> execute as
>  * as single programmed DMA transfer.  On all systems, these messages are
>  * queued, and might complete after transactions to other devices.
> Messages
>  * sent to a given spi_device are alway executed in FIFO order.
> ---------------------------------------------------
> 
> 
> > I don't know, if it is only me, but I would like to see some more abstract
> > description for all
> > these scenarios, before the details of an interface really could be discussed
> > and decided.
> >
> Agree.. Does above matches your requirement ?
> 
Yes, looks better. Thanks!

> 
> with regards, pekon

Best Regards,
Thomas


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