Re: [PATCH] eeprom: at25: convert to spi-mem API

[Christophe Leroy]

Le 08/11/2025 à 12:41, Sverdlin, Alexander a écrit :
> Hi Christophe,
>
> On Sat, 2025-11-08 at 12:14 +0100, Christophe Leroy wrote:
> > > Now I'm trying to understand why the problem surfaced with commit
> > > 8ad6249c51d0 ("eeprom: at25: convert to spi-mem API")
> >
> > The reason why it was not a problem before was that the transfer was
> > done into of->prealloc_buf (fs/kernfs/file.c) which is a kmalloc() with
> > size (PAGE_SIZE + 1).
> >
> > Following the rework of at25 it now goes into the bounce buffer which is
> > allocated with the exact size of the transfer.
> >
> > Why do we need an intermediate bounce buffer now, why can't
> > of->prealloc_buf be used directly as before ?
>
> userspace access is only one part of the story, the other is NVMEM
> kernel-internal API, like nvmem_cell_read*() and I suppose there is
> no requirement for a destination buffer to be DMA-able.

As far as I can see nvmem_cell_read*() allocates a kmalloc() bounce 
buffer already:

	buf = kzalloc(max_t(size_t, entry->raw_len, entry->bytes), GFP_KERNEL);
	if (!buf)
		return ERR_PTR(-ENOMEM);

	rc = __nvmem_cell_read(nvmem, cell->entry, buf, len, cell->id, 
cell->index);
	if (rc) {
		kfree(buf);
		return ERR_PTR(rc);
	}

	return buf;

Christophe