Re: [patch 2.6.28-rc2] at91_mci: workaround lockdep

[Nicolas Ferre <nicolas.ferre@atmel.com>]

David Brownell :
> From: David Brownell <dbrownell@users.sourceforge.net>
> 
> Lockdep reported a problem in the at91_mci driver ... in this case, the
> issue is with lockdep, not with the driver.  A trimmed stack dump, from
> trying to boot with root on MMC, shows:
> 
>   WARNING: at kernel/lockdep.c:2195 trace_hardirqs_on_caller+0xf4/0x170()
>   Modules linked in:
>   [<c005bc98>] (trace_hardirqs_on+0x0/0x18) from [<c0213bf4>] (_spin_unlock_irq+0x2c/0x3c)
>   [<c0213bc8>] (_spin_unlock_irq+0x0/0x3c) from [<c0029a88>] (flush_dcache_page+0x114/0x144)
>   [<c0029974>] (flush_dcache_page+0x0/0x144) from [<c019b034>] (at91_mci_irq+0x150/0x414)
>   [<c019aee4>] (at91_mci_irq+0x0/0x414) from [<c0066c5c>] (handle_IRQ_event+0x2c/0x6c)
>   [<c0066c30>] (handle_IRQ_event+0x0/0x6c) from [<c0068a60>] (handle_level_irq+0x108/0x124)
>   [<c0068958>] (handle_level_irq+0x0/0x124) from [<c0022064>] (__exception_text_start+0x64/0x90)
> 
> When __flush_dcache_aliases() returns -- inlined into flush_dcache_page(),
> above -- it re-enables IRQs ... since that evidently may only be called with
> IRQs enabled.  That's OK since the (unshared) IRQ handler doesn't ask for IRQs
> to be disabled.   Except ... that lockdep went and disabled them, then went on
> to complains about the breakage *it* caused!
> 
> Workaround: depend on LOCKDEP=n ... and for paranoia, disable IRQF_SHARED
> for this interrupt.  (At the hardware level, this is dedicated to MCI, so
> there's never a need for multiple handlers.)
> 
> Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>

> ---
> An alternate fix might be to have ARM dcache mmap locking save and restore
> the IRQ flags ... but there are several such "can't run with IRQs disabled"
> restrictions scattered through that code, with no rationales commented.  Or
> stop lockdep from mucking with IRQ flags; probably not any time soon!
> 
> So for now, this seems to be the best "solution" available...
> 
>  drivers/mmc/host/Kconfig    |    1 +
>  drivers/mmc/host/at91_mci.c |    3 +--
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/drivers/mmc/host/Kconfig
> +++ b/drivers/mmc/host/Kconfig
> @@ -99,6 +99,7 @@ config MMC_AU1X
>  config MMC_AT91
>  	tristate "AT91 SD/MMC Card Interface support"
>  	depends on ARCH_AT91
> +	depends on LOCKDEP=n
>  	help
>  	  This selects the AT91 MCI controller.
>  
> --- a/drivers/mmc/host/at91_mci.c
> +++ b/drivers/mmc/host/at91_mci.c
> @@ -1081,8 +1081,7 @@ static int __init at91_mci_probe(struct 
>  	 * Allocate the MCI interrupt
>  	 */
>  	host->irq = platform_get_irq(pdev, 0);
> -	ret = request_irq(host->irq, at91_mci_irq, IRQF_SHARED,
> -			mmc_hostname(mmc), host);
> +	ret = request_irq(host->irq, at91_mci_irq, 0, mmc_hostname(mmc), host);
>  	if (ret) {
>  		dev_dbg(&pdev->dev, "request MCI interrupt failed\n");
>  		goto fail0;
> 

Thanks, regards,
-- 
Nicolas Ferre