linux-kernel - RE: [PATCH v4 08/11] tty: add pruss SUART driver

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Date:	Mon, 2 May 2011 22:45:53 +0530
From:	"Nori, Sekhar" <nsekhar@...com>
To:	Subhasish Ghosh <subhasish@...tralsolutions.com>,
	Greg KH <gregkh@...e.de>
CC:	Greg KH <greg@...ah.com>,
	"davinci-linux-open-source@...ux.davincidsp.com" 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"Watkins, Melissa" <m-watkins@...com>,
	"sachi@...tralsolutions.com" <sachi@...tralsolutions.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4 08/11] tty: add pruss SUART driver

On Mon, May 02, 2011 at 14:04:11, Subhasish Ghosh wrote:

> >> SG - So, should I call the sram_alloc() in the platform setup function. 
> > 
> > Can you please shed some light on how SRAM
> > is being used in the driver? Looking at the
> > driver, it looks like it is used as a shared
> > buffer between the PRU firmware and kernel.
> > 
> > If yes, how do you cope with dynamic allocation
> > of SRAM? That is, how do you inform the firmware
> > what portion of SRAM has been allocated to the
> > driver?
> > 
> > Also, usage of SRAM is not required for basic driver
> > function, correct? So, a platform which does not
> > have SRAM to spare for this driver could still have
> > a portion of SDRAM/DDR allocated to be used as the
> > shared buffer? I guess SRAM was used only for lower
> > access times. But it should still be possible to
> > sustain lower baudrates with SDRAM/DDR?
> 
> The sram is allocated dynamically in the driver. 
> After allocation, we write the pointer into the PRU, so in case the
> driver allocates memory form the DDR, it will write this info into the
> PRU and it will work. But, because of DDR access latencies, the UART
> will work only for lower baud rates. 

Thanks for the clarification. In this case, the driver
should use platform callbacks to get/put fast fifo
space. In case this callback is not populated by the
platform or returns an error, the driver should fall
back to allocating from DDR.

Thanks,
Sekhar

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