linux-kernel - Re: [PATCH] habanalabs: fix build warning

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Date:   Fri, 1 Apr 2022 08:40:00 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Max Filippov <jcmvbkbc@...il.com>
Cc:     Ohad Sharabi <osharabi@...ana.ai>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] habanalabs: fix build warning

On Fri, Apr 1, 2022 at 6:16 AM Max Filippov <jcmvbkbc@...il.com> wrote:
>
> allmodconfig build fails on ARCH=xtensa with the following message:
>
>   drivers/misc/habanalabs/common/memory.c:153:49: error: cast from pointer
>         to integer of different size [-Werror=pointer-to-int-cast]
>         (u64) gen_pool_dma_alloc_align(vm->dram_pg_pool,
>
> Fix it by adding intermediate conversion to uintptr_t as in other places
> in that driver.
>
> Fixes: e8458e20e0a3 ("habanalabs: make sure device mem alloc is page aligned")
> Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
> ---
>  drivers/misc/habanalabs/common/memory.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/misc/habanalabs/common/memory.c b/drivers/misc/habanalabs/common/memory.c
> index e008d82e4ba3..f0d373171d2a 100644
> --- a/drivers/misc/habanalabs/common/memory.c
> +++ b/drivers/misc/habanalabs/common/memory.c
> @@ -150,9 +150,9 @@ static int alloc_device_memory(struct hl_ctx *ctx, struct hl_mem_in *args,
>                 for (i = 0 ; i < num_pgs ; i++) {
>                         if (is_power_of_2(page_size))
>                                 phys_pg_pack->pages[i] =
> -                                               (u64) gen_pool_dma_alloc_align(vm->dram_pg_pool,
> -                                                                               page_size, NULL,
> -                                                                               page_size);
> +                                       (u64)(uintptr_t)gen_pool_dma_alloc_align(vm->dram_pg_pool,
> +                                                                                page_size, NULL,
> +                                                                                page_size);
>                         else
>                                 phys_pg_pack->pages[i] = (u64) gen_pool_alloc(vm->dram_pg_pool,
>                                                                                 page_size);

This addresses the warning, but I suspect there is still a problem in the code:
The description of that member lists it as '@...es: the physical page array',
but it is actually a kernel virtual address that gets passed to it. Since this
is a 'u64' member, it is hard to tell what type it actually is.

gen_pool_dma_alloc_align() returns both a virtual address and a dma (bus)
address. The dma address is ignored here, which makes me wonder why
this interface is used in the first place.

I can see four possible things that may be going on here:

- if the pages[] array is meant to be a kernel virtual address, it should be
  changed from a 'u64' to a normal pointer, with the cast removed.

- if the pages[] array is meant to be a physical address, as documented,
  it should be assigned using virt_to_phys() on the pointer, with a warning
  that this must not be used a as a dma address (which can easily get
  confused with a phys address as the binary representation is often the
  same in the absence of an iommu). In this case, it should also be
  changed to a phys_addr_t.

- if the pages[] array is meant to be a dma address, it should be changed
  to a dma_addr_t, and passed as the third argument to
  gen_pool_dma_alloc_align() in order to return the correct address.

- if there is a 'u64' member that is used for two (or all three) of the above
  depending on context, it should be replaced with either multiple
  struct members or a union.

Looking at other uses of the pages[] array, I see a dma_addr_t assigned
to it in init_phys_pg_pack_from_userptr(), but map_phys_pg_pack() and
alloc_sgt_from_device_pages appear to treat it as a cpu-physical phys_addr_t
rather than a device address again.

        Arnd