GSOC 2015: Raspberry Pi Low Level Peripherals and SD Card

[André Marques]

On 25-03-2015 14:17, Gedare Bloom wrote:
> On Wed, Mar 25, 2015 at 4:49 AM, André Marques
> <andre.lousa.marques at gmail.com> wrote:
>> Hello everyone,
>>
>> My GSoC proposal entitled "Raspberry Pi Low Level Peripherals and SD Card"
>> can be found for review at:
>>
>> https://docs.google.com/document/d/11K0qU3OsbSMhNYkTTWt4kitAbd6uuJBcMfA_07Lg6j8
>>
>> One issue I would like to discuss here is related to the SD card support,
>> which as exposed on the proposal the GPIOs to which the SD card is connected
>> internally do not have SPI hardware capabilities, meaning that to access the
>> card via SPI mode it would have to be bit-banged on those GPIOs (a problem
>> on performance). This is more detailed on the proposal, but another
>> alternative would be to use the SD mode instead, with the help of the PI's
>> EMMC module, and the SD simplified specifications as documentation for the
>> SD protocol. I have already used this setup with success in the past, and it
>> should have better performance. In this setup the FreeBSD SD/MMC stack may
>> be used (such as the one Sebastian Huber ported last year ->
>> https://git.rtems.org/sebh/rtems-libusb.git/tree/rtems/freebsd/dev/mmc?id=3c82a1500da3192de2504a1360e065fd84a1f3a0)
> Is this code in the libbsd.git now?

It is not, only on Sebastian's repository. It was anounced here along 
with an USB host stack:

https://lists.rtems.org/pipermail/devel/2014-August/007873.html

>
>> which implements the SD protocol also based on the simplified specs. This
>> would help avoiding implementing the protocol from scratch (which is sort of
>> what I did in my previous effort with the SD card and the PI) and would be
>> better for maintenance probably.
>>
>> My issue is then on the feasibily (licence wise) of having the FreeBSD SD
>> stack or having code based on the SD Simplified Specs on the RTEMS tree.
>>
> FreeBSD code should be fine. The complexity of getting libbsd working
> could be the real stumbling block. But I think others will be working
> on the same problem too, so there may be some ability to share
> knowledge.
>
>> In SPI mode an implementation of the protocol can be found in
>> libchip/i2c/spi-sd-card.*, which could in that case be used/improved further
>> during the project, if it is decided it is best to use SPI mode. This uses
>> the libi2c API, which is deprecated for I2C but, as I understand, still
>> stands as the API for SPI (information about that would also be
>> appreciated).
>>
> It's just the no one has replaced it yet! There seem to be multiple
> efforts going on now to get more driver-like frameworks into RTEMS.

If any actual effort is currently going or planned for a new SPI API it 
would be interesting to know so it can be taken into account, so it does 
not come as a surprise mid-GSoC.

>> Thanks,
>> André Marques.
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