linux-kernel - Re: [PATCH v2 1/4] firmware: xilinx: Add sysfs interface

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Date:   Tue, 28 Jan 2020 07:28:14 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jolly Shah <JOLLYS@...inx.com>
Cc:     "ard.biesheuvel@...aro.org" <ard.biesheuvel@...aro.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "matt@...eblueprint.co.uk" <matt@...eblueprint.co.uk>,
        "sudeep.holla@....com" <sudeep.holla@....com>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "dmitry.torokhov@...il.com" <dmitry.torokhov@...il.com>,
        Michal Simek <michals@...inx.com>,
        Rajan Vaja <RAJANV@...inx.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] firmware: xilinx: Add sysfs interface

On Mon, Jan 27, 2020 at 11:01:27PM +0000, Jolly Shah wrote:
>     > > > +	ret = kstrtol(tok, 16, &value);
>     > > > +	if (ret) {
>     > > > +		ret = -EFAULT;
>     > > > +		goto err;
>     > > > +	}
>     > > > +
>     > > > +	ret = eemi_ops->ioctl(0, read_ioctl, reg, 0, ret_payload);
>     > > 
>     > > This feels "tricky", if you tie this to the device you have your driver
>     > > bound to, will this make it easier instead of having to go through the
>     > > ioctl callback?
>     > > 
>     > 
>     > GGS(general global storage) registers are in PMU space and linux doesn't have access to it 
>     > Hence ioctl is used.
>     
>     Why not just a "real" call to the driver to make this type of reading?
>     You don't have ioctls within the kernel for other drivers to call,
>     that's not needed at all.
> 
> these registers are for users  and for special needs where users wants
> to retain values over resets. but as they belong to PMU address space,
> these interface APIs are provided. They don’t allow access to any
> other registers.

That's not the issue here.  The issue is you are using an "internal"
ioctl, instead just make a "real" call.

>     > > > +int zynqmp_pm_ggs_init(struct kobject *parent_kobj)
>     > > > +{
>     > > > +	return sysfs_create_group(parent_kobj, zynqmp_ggs_groups[0]);
>     > > 
>     > > You might be racing userspace here and loosing :(
>     > 
>     > Prob is called before user space is notified about sysfs so racing shouldn't happen.
>     
>     "shouldn't"?  How do you know this?
> 
> Since firmware driver is always built-in (we don't provide support to
> use as module), user space won't be available before prob is complete.
> Correct if I am wrong.

Userspace starts earlier than you think, and also, use the correct
interfaces for this type of thing, that is why it is there.  Don't
create purposfully-incorrect code :)

>     > > > diff --git a/drivers/firmware/xilinx/zynqmp.c
>     > > b/drivers/firmware/xilinx/zynqmp.c
>     > > > index 75bdfaa..4c1117d 100644
>     > > > --- a/drivers/firmware/xilinx/zynqmp.c
>     > > > +++ b/drivers/firmware/xilinx/zynqmp.c
>     > > > @@ -473,6 +473,10 @@ static inline int zynqmp_is_valid_ioctl(u32 ioctl_id)
>     > > >  	case IOCTL_GET_PLL_FRAC_MODE:
>     > > >  	case IOCTL_SET_PLL_FRAC_DATA:
>     > > >  	case IOCTL_GET_PLL_FRAC_DATA:
>     > > > +	case IOCTL_WRITE_GGS:
>     > > > +	case IOCTL_READ_GGS:
>     > > > +	case IOCTL_WRITE_PGGS:
>     > > > +	case IOCTL_READ_PGGS:
>     > > 
>     > > Huh???
>     > 
>     > Sorry not sure about your concern here. These registers are in PMU space and hence
>     > Ioctl is needed to let linux access them.
>     
>     userspace or kernelspace?
>     
>     You seem to be mixing them both here.
> 
> They are in Platform Management Unit register address space so it
> allows only secure access. Hence for linux to access it, interface
> APIs are provided. 

Again, that's fine, but why are you creating an "internal ioctl"?  Just
make a real function call.

thanks,

greg k-h