This is an updated version of the patch from Mark Underwood, handling
the no-memory case better and using SLAB_KERNEL not SLAB_ATOMIC.
Please apply it on top of the current SPI code in the MM tree.
- Dave
So you're turning this to be unsafe if the buffer is in use, right? Funny...
David Brownell wrote:
>This is an updated version of the patch from Mark Underwood, handling
>the no-memory case better and using SLAB_KERNEL not SLAB_ATOMIC.
>
>Please apply it on top of the current SPI code in the MM tree.
>
>- Dave
>
>
>------------------------------------------------------------------------
>
>Update the SPI framework to remove a potential priority inversion case by
>reverting to kmalloc if the pre-allocated DMA-safe buffer isn't available.
>
>From: Mark Underwood <[email protected]>
>Signed-off-by: David Brownell <[email protected]>
>
>--- g26.orig/drivers/spi/spi.c 2005-12-11 11:06:38.000000000 -0800
>+++ g26/drivers/spi/spi.c 2005-12-13 09:56:22.000000000 -0800
>@@ -541,22 +541,30 @@ int spi_write_then_read(struct spi_devic
> int status;
> struct spi_message message;
> struct spi_transfer x[2];
>+ u8 *local_buf;
>
> /* Use preallocated DMA-safe buffer. We can't avoid copying here,
> * (as a pure convenience thing), but we can keep heap costs
>- * out of the hot path.
>+ * out of the hot path ...
> */
> if ((n_tx + n_rx) > SPI_BUFSIZ)
> return -EINVAL;
>
>- down(&lock);
>+ /* ... unless someone else is using the pre-allocated buffer */
>+ if (down_trylock(&lock)) {
>+ local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
>+ if (!local_buf)
>+ return -ENOMEM;
>+ } else
>+ local_buf = buf;
>+
> memset(x, 0, sizeof x);
>
>- memcpy(buf, txbuf, n_tx);
>- x[0].tx_buf = buf;
>+ memcpy(local_buf, txbuf, n_tx);
>+ x[0].tx_buf = local_buf;
> x[0].len = n_tx;
>
>- x[1].rx_buf = buf + n_tx;
>+ x[1].rx_buf = local_buf + n_tx;
> x[1].len = n_rx;
>
> /* do the i/o */
>@@ -568,7 +576,11 @@ int spi_write_then_read(struct spi_devic
> status = message.status;
> }
>
>- up(&lock);
>+ if (x[0].tx_buf == buf)
>+ up(&lock);
>+ else
>+ kfree(local_buf);
>+
> return status;
> }
> EXPORT_SYMBOL_GPL(spi_write_then_read);
>
>
On Tuesday 13 December 2005 1:49 pm, Vitaly Wool wrote:
> So you're turning this to be unsafe if the buffer is in use, right? Funny...
I have no idea what you mean by that comment. The parameters to that
function have always been documented as "will copy", and the two branches
(busy/not) differ only in _which_ buffer they use from the heap (the
fast pre-allocated one, or a freshly allocated scratch buffer). Heap
buffers are by definition DMA-safe.
- Dave
> David Brownell wrote:
>
> >This is an updated version of the patch from Mark Underwood, handling
> >the no-memory case better and using SLAB_KERNEL not SLAB_ATOMIC.
> >
>
Okay, sorry, I've misundastood that.
Vitaly
David Brownell wrote:
>On Tuesday 13 December 2005 1:49 pm, Vitaly Wool wrote:
>
>
>>So you're turning this to be unsafe if the buffer is in use, right? Funny...
>>
>>
>
>I have no idea what you mean by that comment. The parameters to that
>function have always been documented as "will copy", and the two branches
>(busy/not) differ only in _which_ buffer they use from the heap (the
>fast pre-allocated one, or a freshly allocated scratch buffer). Heap
>buffers are by definition DMA-safe.
>
>- Dave
>
>
>
>
>>David Brownell wrote:
>>
>>
>>
>>>This is an updated version of the patch from Mark Underwood, handling
>>>the no-memory case better and using SLAB_KERNEL not SLAB_ATOMIC.
>>>
>>>
>>>
>
>
>
>
Hmm...
>- down(&lock);
>+ /* ... unless someone else is using the pre-allocated buffer */
>+ if (down_trylock(&lock)) {
>+ local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
>+ if (!local_buf)
>+ return -ENOMEM;
>+ } else
>+ local_buf = buf;
>+
>
>
Okay, so suppose we have two controller drivers working in two threads
and calling write_then_read in such a way that the one called later has
to allocate a new buffer. Suppose also that both controller drivers are
working in PIO mode. In this situation you have one redundant kmalloc
and two redundant memcpy's, not speaking about overhead brought up by
mutexes. Bad!
So I still can't say I'm accepting this approach.
Worth mentioning is that the approach I propose is free from this drawback.
Vitaly