GSOC 2015: Raspberry Pi Low Level Peripherals and SD Card

[Gedare Bloom]

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?

> 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.

> Thanks,
> André Marques.
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