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

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Message-ID: <4EF659A1-2844-46B9-9ED6-5A6A20401D9D@xilinx.com>
Date:   Thu, 30 Jan 2020 23:59:03 +0000
From:   Jolly Shah <JOLLYS@...inx.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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

Hi Greg,

On 1/27/20, 10:28 PM, "linux-kernel-owner@...r.kernel.org on behalf of Greg KH" <linux-kernel-owner@...r.kernel.org on behalf of gregkh@...uxfoundation.org> wrote:

    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.

Sorry I am not clear. Do you mean that we should use linux standard ioctl interface instead of internal ioctl by mentioning "real" ?
    
    >     > > > +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 :)

Sure. We will change it.

Thanks,
Jolly Shah

    
    >     > > > 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