linux-kernel - Re: [PATCH] tty: amba-pl011: Use 32-bit accesses for SBSA UART

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Date:	Wed, 30 Mar 2016 09:55:51 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Christopher Covington <cov@...eaurora.org>
Cc:	Andre Przywara <andre.przywara@....com>,
	Russell King <linux@....linux.org.uk>,
	Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, Timur Tabi <timur@...eaurora.org>,
	Jon Masters <jcm@...hat.com>,
	Mark Langsdorf <mlangsdo@...hat.com>,
	Dave Martin <dave.martin@...aro.org>,
	Aleksey Makarov <aleksey.makarov@...aro.org>
Subject: Re: [PATCH] tty: amba-pl011: Use 32-bit accesses for SBSA UART

On Wed, Mar 30, 2016 at 08:30:34AM -0400, Christopher Covington wrote:
> Hi Greg,
> 
> On 03/15/2016 06:08 AM, Andre Przywara wrote:
> > Hi Christopher,
> > 
> > On 11/03/16 06:35, Christopher Covington wrote:
> >> Version 2 of the Server Base System Architecture (SBSAv2) describes the
> >> Generic UART registers as 32 bits wide. At least one implementation, found
> >> on the Qualcomm Technologies QDF2432, only supports 32 bit accesses.
> >> SBSAv3, which describes supported access sizes in greater detail,
> >> explicitly requires support for both 16 and 32 bit accesses to all
> >> registers (and 8 bit accesses to some but not all). Therefore, for broad
> >> compatibility, simply use 32 bit accessors for the SBSA UART.
> >>
> >> Tested-by: Mark Langsdorf <mlangsdo@...hat.com>
> >> Signed-off-by: Christopher Covington <cov@...eaurora.org>
> > 
> > So I gave this a try on a Juno and a Midway. Both have a normal PL011,
> > but I changed the DT to advertise an SBSA UART instead.
> > This worked fine with the 32bit accessors.
> > Also according to some research on the hardware size at least the
> > current ARM PL011 implementation are totally fine with 32-bit (as well
> > as 16-bit) accesses.
> > There is some reluctance about whether this is true for _every_ older
> > PL011 implementation, but they are out of scope here, as we are talking
> > about the SBSA only.
> > So:
> > 
> > Tested-by: Andre Przywara <andre.przywara@....com>
> > Acked-by: Andre Przywara <andre.przywara@....com>
> > 
> > You can add Juno and Midway to the list of tested systems.
> 
> >> Changes new in v2:
> 
> Apologies for omitting the v2 prefix in the second version of the patch
> that I sent out.
> 
> >> * Fixed from address
> >> * Elaborated on forward (SBSAv3) compatibility in commit message
> >> * Included Mark Langsdorf's Tested-by, which now covers:
> >>     QDF2432
> >>     Seattle
> >>     X-Gene 1
> 
> >> ---
> >>  drivers/tty/serial/amba-pl011.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> >> index c0da0cc..ffb5eb8 100644
> >> --- a/drivers/tty/serial/amba-pl011.c
> >> +++ b/drivers/tty/serial/amba-pl011.c
> >> @@ -121,6 +121,7 @@ static struct vendor_data vendor_arm = {
> >>  
> >>  static struct vendor_data vendor_sbsa = {
> >>  	.reg_offset		= pl011_std_offsets,
> >> +	.access_32b		= true,
> >>  	.oversampling		= false,
> >>  	.dma_threshold		= false,
> >>  	.cts_event_workaround	= false,
> >>
> 
> Do you consider this patch suitable to be included in a 4.6 release
> candidate? It fixes an issue running this driver on certain hardware,
> and with gracious assistance we've performed due diligence to check that
> it does not adversely affect the driver running on other hardware. Would
> it be useful to send a v3 collecting the acks and tested-bys?

If this isn't a bug fix or regression fix, it's not ok for 4.6-final, it
will have to wait for 4.7-rc1.

And yes, collecting the acks and tested-bys would be great, I'm always
glad to see that, it makes my job easier.  Now that 4.6-rc1 is out, I'll
start to dig through the list of pending patches here, give me a few
weeks to get all of them, especially due to the conference travel I'm
currently doing...

thanks,

greg k-h