netdev - Re: [PATCH] r8169: default to 64-bit DMA on systems without memory below 4 GB

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Date:	Wed, 11 May 2016 22:34:13 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Francois Romieu <romieu@...zoreil.com>
Cc:	"<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
	Ricardo Salveti <ricardo.salveti@...aro.org>,
	Leo Duran <leo.duran@....com>,
	G Gregory <graeme.gregory@...aro.org>,
	Realtek linux nic maintainers <nic_swsd@...ltek.com>
Subject: Re: [PATCH] r8169: default to 64-bit DMA on systems without memory
 below 4 GB

On 11 May 2016 at 22:31, Francois Romieu <romieu@...zoreil.com> wrote:
> Ard Biesheuvel <ard.biesheuvel@...aro.org> :
>> The current logic around the 'use_dac' module parameter prevents the
>> r81969 driver from being loadable on 64-bit systems without any RAM
>> below 4 GB when the parameter is left at its default value.
>> So introduce a new default value -1 which indicates that 64-bit DMA
>> should be enabled implicitly, but only if setting a 32-bit DMA mask
>> has failed earlier. This should prevent any regressions like the ones
>> caused by previous attempts to change this code.
>
> I am not a huge fan but if you really need it...
>
> Which current kernel arches do exhibit the interesting
> f*ck-legacy-32-bits-only-devices property you just described ?
>

It has little to do with f*cking legacy 32-bits-only-devices if DRAM
simply starts at 0x80_0000_0000. This is on an AMD arm64 chip.

> [...]
>> diff --git a/drivers/net/ethernet/realtek/r8169.c b/drivers/net/ethernet/realtek/r8169.c
>> index 94f08f1e841c..a49e8a58e539 100644
>> --- a/drivers/net/ethernet/realtek/r8169.c
>> +++ b/drivers/net/ethernet/realtek/r8169.c
> [...]
>> @@ -859,7 +859,8 @@ struct rtl8169_private {
>>  MODULE_AUTHOR("Realtek and the Linux r8169 crew <netdev@...r.kernel.org>");
>>  MODULE_DESCRIPTION("RealTek RTL-8169 Gigabit Ethernet driver");
>>  module_param(use_dac, int, 0);
>> -MODULE_PARM_DESC(use_dac, "Enable PCI DAC. Unsafe on 32 bit PCI slot.");
>> +MODULE_PARM_DESC(use_dac,
>> +     "Enable PCI DAC. Unsafe on 32 bit PCI slot (default -1: enable on 64-bit archs only if needed");
>
> Nit: the parameter is bizarre enough that you could leave the original
> description.
>

OK, if you prefer. Should I send a v2?