linux-kernel - Re: [PATCH] usb: musb: USB_TI_CPPI41

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Date:	Wed, 18 Nov 2015 14:39:10 -0600
From:	Bin Liu <binmlist@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	George Cherian <george.cherian@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	balbi@...com
Subject: Re: [PATCH] usb: musb: USB_TI_CPPI41_DMA requires dmaengine support

Hi,

On Wed, Nov 18, 2015 at 2:27 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Wednesday 18 November 2015 12:29:27 Bin Liu wrote:
>> > diff --git a/drivers/usb/musb/Kconfig b/drivers/usb/musb/Kconfig
>> > index 1f2037bbeb0d..45c83baf675d 100644
>> > --- a/drivers/usb/musb/Kconfig
>> > +++ b/drivers/usb/musb/Kconfig
>> > @@ -159,7 +159,7 @@ config USB_TI_CPPI_DMA
>> >
>> >  config USB_TI_CPPI41_DMA
>> >         bool 'TI CPPI 4.1 (AM335x)'
>> > -       depends on ARCH_OMAP
>> > +       depends on ARCH_OMAP && DMADEVICES
>> >         select TI_CPPI41
>>
>> I am not sure what the generic policy is, but instead of hiding
>> USB_TI_CPPI41_DMA if DMADEVICES is disabled, I'd like to enable
>> DMADEVICES if USB_TI_CPPI41_DMA is enabled, from user experience
>> perspective.
>
> General policy is that you should not 'select' a symbol that is
> also user-visible, as that tends to cause dependency loops and
> other problems when something is enabled without the user being
> aware of that.

Understood. Thanks. I am okay with this patch.

>
> Ideally we should remove the 'select TI_CPPI41' here as well, but
> what we could do instead is to make that a silent symbol and remove
> the prompt so it always gets enabled implicitly when USB_TI_CPPI41_DMA
> and DMADEVICES are both enabled.

But what if DMADEVICES was disabled and USB_TI_CPPI41_DMA was enabled?
I would think I had CPPI fully enabled for MUSB, but it didn't because
TI_CPPI41 was disabled.

I would think this patch is the test option so far, we might have to
document somewhere that to dmaengine has to be enabled to use MUSB
CPPI, but I am not sure where the best place is to document...

Regards,
-Bin.

>
>         Arnd
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