linux-kernel - Re: [patch 2.6.23-rc1] dma_free_coherent() needs irqs enabled (sigh)

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Date:	Tue, 24 Jul 2007 23:07:38 +0100
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	David Brownell <david-b@...bell.net>
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [patch 2.6.23-rc1] dma_free_coherent() needs irqs enabled (sigh)

On Tue, Jul 24, 2007 at 02:29:05PM -0700, David Brownell wrote:
> On at least ARM (and I'm told MIPS too) dma_free_coherent() has a newish
> call context requirement:  unlike its dma_alloc_coherent() sibling, it
> may not be called with IRQs disabled.  (This was new behavior on ARM as
> of late 2006, caused by ARM SMP updates.)

I think you got the year wrong:

5edf71ae (Russell King      2005-11-25 15:52:51 +0000 364)      WARN_ON(irqs_disabled());

which is due to this commit:

[ARM] Do not call flush_tlb_kernel_range() with IRQs disabled.

We must not call TLB maintainence operations with interrupts disabled,
otherwise we risk a lockup in the SMP IPI code.

This means that consistent_free() can not be called from a context with
IRQs disabled.  In addition, we must not hold the lock in consistent_free
when we call flush_tlb_kernel_range().  However, we must continue to
prevent consistent_alloc() from re-using the memory region until we've
finished tearing down the mapping and dealing with the TLB.

Therefore, leave the vm_region entry in the list, but mark it inactive
before dropping the lock and starting the tear-down process.  After the
mapping has been torn down, re-acquire the lock and remove the entry
from the list.

Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>

> Since it looks like that restriction won't be removed, this patch changes
> the definition of the API to include that requirement.

The PCI DMA-mapping API had this restriction.  For some reason, this
restriction was not carried forward into the DMA-API.  Unfortunately
the restriction can not be removed without causing the problems
described in the commit which introduced it.

Or alternatively we scrap ARM SMP entirely, which isn't going to happen.

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