Re: [PATCH 1/3] mtd: spi-nor: add optional DMA-safe bounce buffer for data transfer

[Trent Piepho <tpiepho@impinj.com>]

On Fri, 2017-12-29 at 15:46 +0530, Vignesh R wrote:
> On Friday 29 December 2017 12:24 AM, Trent Piepho wrote:
> > 
> > > Vignesh has suggested to call virt_addr_valid() instead.
> > > I think Boris has also told me about this function.
> > > So it might be the right solution. What do you think about their proposal?
> > 
> > Not sure what exactly the differences are between these methods.  The
> > fact that each of the many existing DMA fixes uses slightly different
> > code to detect what is unsafe speaks to the difficulty of this problem!
> 
> My understanding based on Documentation/DMA-API-HOWTO.txt and
> Documentation/arm/memory.txt is that
> virt_addr_valid() will guarantee that address is in range of
> PAGE_OFFSET to high_memory-1 (Kernel direct-mapped RAM region) which is
> address range of buffers that are DMA'able.

There's code in gpmi-nand.c that does:

        /* first try to map the upper buffer directly */
        if (virt_addr_valid(this->upper_buf) &&
                !object_is_on_stack(this->upper_buf)) {
                sg_init_one(sgl, this->upper_buf, this->upper_len);

So whoever wrote that thought that stack objects needed an additional
test beyond virt_addr_valid.  But it does appear to be far more common
to depend on just virt_addr_valid, so perhaps the code in gpmi-nand is
in error.

> >  virt_addr_valid() is already used by spi-ti-qspi.  spi core uses for
> > the buffer map helper, but that code path is for buffers which are NOT
> > vmalloc or highmem, but are still not virt_addr_valid() for some other
> > reason.
> > 
> 
> 	if (vmalloced_buf || kmap_buf) {
> 		/* Handle vmalloc'd or kmap'd buffers */
> 		...
This stuff does get DMAed.  So I have to wonder, if spi.c thinks it can
use DMA with vmalloc or highmem, couldn't spi-not do the same instead
of the bounce buffer?

>         } else if (virt_addr_valid(buf)) {
> 		/* Handle kmalloc'd and such buffers */
>                 ...
> 	} else {
> 		/* Error if none of the above */

So what is this case here for?  It's some class that does not have a
valid virtual address and yet is not vmalloc or highmem.

> 		return -EINVAL;
> 	}
>