Re: MMC Driver RFC

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Wed, Jan 12, 2005 at 09:10:12PM -0000, Richard Purdie wrote:
> 1. Card Detection Interrupts
> 
> The MMC code completely misses card removals on my test hardware as when the 
> interrupt triggers electrical contact is still fully functional.
> 
> The PXA code calls mmc_detect_change() whenever an interrupt is detected. 
> The MMC core then does a schedule_work(&host->detect). The problem is that 
> when the interrupt is generated, the card may not be 100% inserted or 100% 
> removed. Given the mechanical nature of insertions and removals, electrical 
> contact is possible for a while after removal has been started (and a while 
> before insertion is complete).

If your socket works like that, you need to handle that by using a timer
yourself.  It normally only affects removal rather than insertion.

> 2. Card Initialisation Problems
> 
> One of my cards works fine. The other works when I enable debug and doesn't 
> when I don't. I suspect the delay while it does a printk gives something 
> time to happen that doesn't normally when running at full speed!

Different cards behave differently.  I suspect you have yet another
quirky card.

> I suspect this is related to the 1ms wait that was added to mmc_setup() as 
> per comments. Is there any documentation which tells us exactly what timings 
> we should be aiming for here? Has anyone else has problems like this?

There isn't any 1ms wait in mmc_setup().

> 3. SD Support
> 
> Ian Molton seems to have added SD support to handhelds.org's kernel. I'm 
> still trying to contact him to discuss this but the following patch enables 
> SD cards to work for me:
> http://www.rpsys.net/openzaurus/2.6.11-rc1/mmc_sd-r1.patch

People are nervous about SD support - the SD forum has been traditionally
rather closed, and there is the perception that a SD card driver may not
go down well.  I have even heard rumours of patent issues/IP issues in
this area, and I don't wish to get stung.

However, that said, the situation has improved recently - we've gone from
no documentation to limited documentation.  However, this documentation
still isn't sufficient to do the job - eg, the SD formats for the CID
and CSD registers remain completely undocumented.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core