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

[R, Vignesh]

On 8/4/2015 9:21 PM, Mark Brown wrote:
> On Mon, Aug 03, 2015 at 10:27:19AM +0530, Vignesh R wrote:
> 
>> @use_mmap_mode: Some SPI controller chips are optimized for interacting
>> with serial flash memories. These chips have memory mapped interface,
>> through which entire serial flash memory slave can be read/written as if
>> though they are physical memories (like RAM). Using this interface,
>> flash can be accessed using memcpy() function and the spi controller
>> hardware will take care of communicating with serial flash over SPI.
>> Setting this flag will indicate the SPI controller driver that the
>> spi_message is from mtd layer to read from/write to flash. The SPI
>> master driver can then appropriately switch the controller to memory
>> mapped interface to read from/write to flash, based on this flag (See
>> drivers/spi/spi-ti-qspi.c for example).
>> NOTE: If the SPI controller chip lacks memory mapped interface, then the
>> driver will ignore this flag and use normal SPI protocol to read
>> from/write to flash. Communication with non-flash SPI devices is not
>> possible using the memory mapped interface.
> 
> I still can't tell from the above what this interface is supposed to do.
> It sounds like the use of memory mapped mode is supposed to be
> transparent to users, it should just affect how the controller interacts
> with the hardware, but if that's the case why do we need to expose it to
> users at all?  Shouldn't the driver just use memory mapped mode if it's
> faster?
> 

TI QSPI controller has two blocks:
1. SPI_CORE: This is generic(normal) spi mode. This can be used to
communicate with any SPI devices (serial flashes as well as non-flash
devices like touchscreen).
2. SFI_MM_IF(SPI memory mapped interface): The SFI_MM_IF block only
allows reading and writing to an SPI flash device only. Used to speed up
flash reads. It _cannot_ be used to communicate with non flash devices.
Now, the spi_message that ti-qspi receives in transfer_one() callback
can be from mtd device(in which case SFI_MM_IF can be used) or from any
other non flash SPI device (in which case SFI_MM_IF must not be used
instead SPI_CORE is to be used) but there is no way(is there?) to
distinguish where spi_message is from. Therefore I introduced flag
(use_mmap_mode) to struct spi_message. mtd driver will set flag to true,
this helps the ti-qspi driver to determine that the user is flash device
and thus can do read via SFI_MM_IF. If this flag is not set then the
user is assumed to be non flash SPI driver and will use SPI_CORE block
to communicate.

On the whole, I just need a way to determine that the user is a flash
device in order to switch to memory mapped interface.

Regards
Vignesh