linux-kernel - Re: [RFC PATCH 1/5] spi: introduce flag for memory mapped read

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Date:	Wed, 5 Aug 2015 11:05:17 +0530
From:	Vignesh R <vigneshr@...com>
To:	Michal Suchanek <hramrach@...il.com>
CC:	Mark Brown <broonie@...nel.org>,
	devicetree <devicetree@...r.kernel.org>,
	Brian Norris <computersforpeace@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>,
	MTD Maling List <linux-mtd@...ts.infradead.org>,
	<linux-omap@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 1/5] spi: introduce flag for memory mapped read



On 08/05/2015 10:51 AM, Michal Suchanek wrote:
> Hello,
> 
> On 4 August 2015 at 19:59, R, Vignesh <vigneshr@...com> wrote:
>>
>>
>> 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.
>>
> 
> Maybe it can be set on the SPI slave rather than each message.

You mean to add flag to spi_device struct? That's ok for me.

-- 
Regards
Vignesh
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