netdev - Re: ethernet/sfc/ warnings with 32-bit dma_addr

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Date:   Wed, 19 Aug 2020 11:37:00 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Randy Dunlap <rdunlap@...radead.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Michael Brown <mbrown@...systems.co.uk>,
        Solarflare linux maintainers <linux-net-drivers@...arflare.com>,
        Martin Habets <mhabets@...arflare.com>
Subject: Re: ethernet/sfc/ warnings with 32-bit dma_addr_t

On 19/08/2020 01:28, Randy Dunlap wrote:
> Hi,
>
> Does the drivers/net/ethernet/sfc/sfc driver require (expect)
> dma_addr_t to be 64 bits (as opposed to 32 bits)?
>
> I see that several #defines in ef100_regs.h are 64...
>
> When used with DMA_BIT_MASK(64), does the value just need to be
> truncated to 32 bits?  Will that work?
As far as I can tell, truncation to 32 bits is harmless — the
 called function (efx_init_io) already tries every mask from the
 passed one down to 32 bits in case of PCIe hardware limitations.

The ef10 and siena versions also truncate like this (their
 #defines are 48 and 46 respectively), but because they are
 handled indirectly through efx_nic_type, the compiler isn't able
 to determine this statically as it can with ef100.
> When I build this driver on i386 with 32-bit dma_addr_t, I see
> the following build warnings:
Could you test whether explicitly casting to dma_addr_t suppresses
 the warnings?  I.e.

    efx_init_io(efx, bar,
                (dma_addr_t)DMA_BIT_MASK(ESF_GZ_TX_SEND_ADDR_WIDTH),
                pci_resource_len(efx->pci_dev, bar));

-ed