Re: [PATCH] staging: comedi: comedi_isadma: Use a non-NULL device for DMA API

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On 25/04/2019 18:13, Greg Kroah-Hartman wrote:
On Thu, Apr 25, 2019 at 05:26:44PM +0100, Ian Abbott wrote:
The "comedi_isadma" module calls `dma_alloc_coherent()` and
`dma_free_coherent()` with a NULL device pointer which is no longer
allowed.  If the `hw_dev` member of the `struct comedi_device` has been
set to a valid device, that can be used instead.  Unfortunately, all the
current users of the "comedi_isadma" module leave the `hw_dev` member
set to NULL.  In that case, use a static dummy fallback device structure
with the coherent DMA mask set to the ISA bus limit of 16MB.

Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
---
  drivers/staging/comedi/drivers/comedi_isadma.c | 15 +++++++++++++--
  drivers/staging/comedi/drivers/comedi_isadma.h |  3 +++
  2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/staging/comedi/drivers/comedi_isadma.c b/drivers/staging/comedi/drivers/comedi_isadma.c
index b77dc8d5d3ff..8929952516a1 100644
--- a/drivers/staging/comedi/drivers/comedi_isadma.c
+++ b/drivers/staging/comedi/drivers/comedi_isadma.c
@@ -14,6 +14,16 @@
#include "comedi_isadma.h" +/*
+ * Fallback device used when hardware device is NULL.
+ * This can be removed after drivers have been converted to use isa_driver.
+ */
+static struct device fallback_dev = {
+	.init_name = "comedi_isadma fallback device",
+	.coherent_dma_mask = DMA_BIT_MASK(24),
+	.dma_mask = &fallback_dev.coherent_dma_mask,
+};

Ick, no, static struct device are a very bad idea as this is a reference
counted structure and making it static can cause odd problems.

This was based on the use of `struct device x86_dma_fallback_dev` in "arch/x86/kernel/pci-dma.c", and `static struct device isa_dma_dev` in "arch/arm/kernel/dma-isa.c", but perhaps it is not appropriate in non-arch code.

Why not just create a "real" one?  Or better yet, use the real device
for the comedi device as all of these drivers should have one now.

I suppose I could use the comedi class device pointed to by the `class_dev` member of `struct comedi_device` (although that could also be NULL because the comedi core does not currently treat `device_create()` failures as fatal).


thanks,

greg k-h

Thanks for the review,

Ian Abbott.

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