linux-kernel - RE: [PATCH v3 2/2] fpga: zynq-fpga: Add support for readback

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Date:   Wed, 25 Jul 2018 09:12:21 +0000
From:   Appana Durga Kedareswara Rao <appanad@...inx.com>
To:     Alan Tull <atull@...nel.org>
CC:     Moritz Fischer <moritz.fischer@...us.com>,
        Nava kishore Manne <navam@...inx.com>,
        Michal Simek <michals@...inx.com>,
        "linux-fpga@...r.kernel.org" <linux-fpga@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3 2/2] fpga: zynq-fpga: Add support for readback

Hi Alan,

	Thanks for the review...

<Snip> 
> >
> > <Snip>
> >> > +static bool readback_type;
> >> > +module_param(readback_type, bool, 0644);
> >> > +MODULE_PARM_DESC(readback_type,
> >> > +               "readback_type 0-configuration register read "
> >> > +               "1- configuration data read (default: 0)");
> >>
> >> Not sure a module param is the best solution here.
> >
> > Need to provide flexibility to the user to switch between reading of FPGA
> registers and configuration data.
> 
> I suggested calling the attribute 'image' because I thought it would always be
> a read back of the FPGA image information.  I don't think that a sysfs
> attribute's function should change. :)

In Zynq Case it supports two types of the readback (Configuration registers, Configuration data(fpga image))  which may not be the same case for other vendors. 
Since I need to support both the use cases I have differentiated them using module param in the zynq-fpga driver.

As you said we shouldn't change the attribute's function, So in the zynq-fpga driver,
I am planning to add another debugfs attribute for reading back of configuration registers as it is vendor specific readback type.
 (Configuration registers                   ----  Usage:  /sys/kernel/debug/fpga/zynq-fpga/cfgreg_summary) 
(Configuration data(fpga image)    ----   Usage: /sys/kernel/debug/fpga/fpga0/image)
Please let me know your thoughts for the same... 

> 
> > One other option is sysfs. Do you want me to implement sysfs path???
> > Any other suggestions???
> 
> You could implement another sysfs attribute for reading FPGA registers
> with a separate ops callback.   Please clarify what it is doing (not
> all FPGAs have this function) and what that will look like when the user reads
> the from the sysfs

AFAIK we shouldn't add another sysfs/debugfs attribute for vendor-specific readback type in the FPGA manager ops.
Please correct me if my understanding is wrong. 

Regards,
Kedar.

> 
> Alan