Passing NULL dev to dma_alloc_coherent() allowed or not?

[Richard Weinberger <richard@nod.at>]

Hi!

While porting on an old out-of-tree driver I noticed that dma_alloc_coherent()
was used with dev being NULL.

commit 148a97d5a02a62f81b5d6176f871c94a65e1f3af
Author: Dan Carpenter <dan.carpenter@oracle.com>
Date:   Wed Apr 24 17:24:37 2019 +0300

    dma-mapping: remove an unnecessary NULL check
    
    We already dereferenced "dev" when we called get_dma_ops() so this NULL
    check is too late.  We're not supposed to pass NULL "dev" pointers to
    dma_alloc_attrs().
    
    Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
    Signed-off-by: Christoph Hellwig <hch@lst.de>

says that dma_alloc_attrs() with dev being NULL is not allowed, but in
include/linux/dma-mapping.h we have:

static inline void *dma_alloc_coherent(struct device *dev, size_t size,
                dma_addr_t *dma_handle, gfp_t gfp)
{

        return dma_alloc_attrs(dev, size, dma_handle, gfp,
                        (gfp & __GFP_NOWARN) ? DMA_ATTR_NO_WARN : 0);
}

In Linus' tree I see at least three callers of dma_alloc_coherent() with a NULL device.
drivers/staging/emxx_udc/emxx_udc.c:2596:                       ep->virt_buf = dma_alloc_coherent(NULL, PAGE_SIZE,
drivers/tty/synclink.c:3667:            info->buffer_list = dma_alloc_coherent(NULL, BUFFERLISTSIZE, &info->buffer_list_dma_addr, GFP_KERNEL);
drivers/tty/synclink.c:3777:                    BufferList[i].virt_addr = dma_alloc_coherent(NULL, DMABUFFERSIZE, &BufferList[i].dma_addr, GFP_KERNEL);

I think these callers are wrong.
Can you please clarify?

Thanks,
//richard
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