[U-Boot] [v2 2/6] spi: cadence_qspi: remove sram polling from flash read

[Marek Vasut]

On Thursday, August 13, 2015 at 11:04:59 PM, vikas wrote:
> Hi Marek,

Hi!

> On 08/13/2015 01:35 PM, Marek Vasut wrote:
> > On Thursday, August 13, 2015 at 09:49:49 PM, vikas wrote:
> > 
> > Hi!
> > 
> >>>> On 08/12/2015 07:09 PM, Marek Vasut wrote:
> >>>>> On Thursday, July 16, 2015 at 04:27:30 AM, Vikas Manocha wrote:
> >>>>>> There is no need to check for sram fill level. If sram is empty, cpu
> >>>>>> will go in the wait state till the time data is available from
> >>>>>> flash.
> >>>>> 
> >>>>> Consider the following scenario:
> >>>>> - CPU core reads some memory area, but there are no data available
> >>>>> just yet
> >>>>> 
> >>>>>   => CPU core goes into wait state
> >>>>> 
> >>>>> - The data never arrive
> >>>>> 
> >>>>> Will the CPU be stuck forever ? If we checked the fill level first
> >>>>> instead, we would never enter such stuck-state.
> >>>> 
> >>>> This indirect mode of reading/writing would be entered when the
> >>>> read/write addresses are in the programmed valid range of addresses.
> >>>> 
> >>>> Even in case of "data never arrive" scenario, a simple timeout seems
> >>>> better then currently implemented read sram level with timeout.
> >>> 
> >>> How do you implement a "simple timeout" if the CPU core is stuck and
> >>> does not execute instructions ? If you mean interrupt, then forget it,
> >>> U-Boot does not do interrupts ;-)
> >> 
> >> Oh yes, you are right.
> > 
> > So shall we keep the SRAM piece ?
> 
> Although in this case the better solution would be to have watermark
> interrupt/status check based on sram fill level, let us keep the existing
> piece of SRAM.

Good.

> Can we make it configurable (SRAM Level test or not) like from DT or
> #define ?

How would you call such an option ? Something like CONFIG_SYS_YOLO (to indicate
that life is too short to use correct, but slower code) ? :-)

I don't want to have two different codepaths in the codebase, one of which is
buggy. So no, I disagree we should add this option. I also don't think it would
be such a performance improvement, so I only see downsides in such a code.

Best regards,
Marek Vasut