linux-kernel - Re: [PATCH v6 1/4] uio: introduce UIO_MEM_DMA

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Message-ID: <71178af5-b52f-4d60-a438-1cf6b0ecb4d8@roeck-us.net>
Date: Fri, 22 Mar 2024 08:23:13 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Chris Leech <cleech@...hat.com>, Nilesh Javali <njavali@...vell.com>,
 Christoph Hellwig <hch@....de>, John Meneghini <jmeneghi@...hat.com>,
 Lee Duncan <lduncan@...e.com>, Mike Christie <michael.christie@...cle.com>,
 Hannes Reinecke <hare@...nel.org>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
 GR-QLogic-Storage-Upstream@...vell.com, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH v6 1/4] uio: introduce UIO_MEM_DMA_COHERENT type

On 3/22/24 07:30, Greg Kroah-Hartman wrote:
> On Fri, Mar 22, 2024 at 07:16:19AM -0700, Guenter Roeck wrote:
>> On Mon, Feb 05, 2024 at 12:01:37PM -0800, Chris Leech wrote:
>>> Add a UIO memtype specifically for sharing dma_alloc_coherent
>>> memory with userspace, backed by dma_mmap_coherent.
>>>
>>> This is mainly for the bnx2/bnx2x/bnx2i "cnic" interface, although there
>>> are a few other uio drivers which map dma_alloc_coherent memory and will
>>> be converted to use dma_mmap_coherent as well.
>>>
>>> Signed-off-by: Nilesh Javali <njavali@...vell.com>
>>> Signed-off-by: Chris Leech <cleech@...hat.com>
>>> ---
>>
>> Building i386:allyesconfig ... failed
>> --------------
>> Error log:
>> drivers/uio/uio.c: In function 'uio_mmap_dma_coherent':
>> drivers/uio/uio.c:795:16: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
>>    795 |         addr = (void *)mem->addr;
>>        |                ^
> 
> So on 32bit systems phys_addr_t != the same size as (void *)?  How is
> that possible?  We also are doing an explicit cast here, how does this
> not work?

phys_addr_t is not always equivalent to the size of a pointer.
PHYS_ADDR_T_64BIT is a configuration option, after all, and it
is independent of "config 64BIT".

> 
> Ah, do you have CONFIG_X86_PAE enabled?  That would cause that mess,
> ick.
> 
> 
>> cc1: all warnings being treated as errors
>> make[5]: [scripts/Makefile.build:244: drivers/uio/uio.o] Error 1 (ignored)
>> drivers/uio/uio_dmem_genirq.c: In function 'uio_dmem_genirq_open':
>> drivers/uio/uio_dmem_genirq.c:63:39: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
>>     63 |                 uiomem->addr = addr ? (phys_addr_t) addr : DMEM_MAP_ERROR;
>>        |                                       ^
>> drivers/uio/uio_dmem_genirq.c: In function 'uio_dmem_genirq_release':
>> drivers/uio/uio_dmem_genirq.c:92:43: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
>>     92 |                                           (void *) uiomem->addr,
>>        |                                           ^
>> cc1: all warnings being treated as errorsphys_addr_t
>> make[5]: [scripts/Makefile.build:244: drivers/uio/uio_dmem_genirq.o] Error 1 (ignored)
>> drivers/uio/uio_pruss.c: In function 'pruss_probe':
>> drivers/uio/uio_pruss.c:194:34: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
>>    194 |                 p->mem[2].addr = (phys_addr_t) gdev->ddr_vaddr;
>>        |                                  ^
>> cc1: all warnings being treated as errors
>>
>> Caused by this patch and "uio_dmem_genirq: UIO_MEM_DMA_COHERENT conversion" as well
>> as "uio_pruss: UIO_MEM_DMA_COHERENT conversion".
>>
>> I'd suggest to make uio dependent on 64 bit if 32 bit is no longer supported
>> to prevent waste of test builds resources.
> 
> Perhaps disable it if PHYS_ADDR_T_64BIT is not enabled?
> 

i386:allyesconfig sets

CONFIG_X86_HAVE_PAE=y
CONFIG_X86_PAE=y
CONFIG_PHYS_ADDR_T_64BIT=y

so that would not really help. The problem here is that this is a 32-bit build,
meaning pointers are 32 bit, while phys_addr_t is 64 bit. I did not check the
code, but it will simply not work if the pointer is supposed to reflect a
physical address. The new dependency would have to check if sizeof(void *) is
larger or equal to sizeof(phys_addr_t). Even then the code would need a double
cast since it is also possible that sizeof(void *) > sizeof(phys_addr_t).

Guenter