Re: [PATCH 1/1] staging: vchiq: Replace global state with per device state

From: Ojaswin Mujoo
Date: Mon Nov 01 2021 - 13:31:51 EST


On Mon, Nov 01, 2021 at 01:19:17PM +0100, Stefan Wahren wrote:
> Hi Ojaswin,
>
> Am 01.11.21 um 12:09 schrieb Ojaswin Mujoo:
> > Currently, the driver has a global g_state variable which is initialised
> > during probe and directly used all over the driver code. However, this
> > prevents the driver to support multiple VideoCore VPUs at the same time.
> >
> > Replace this global state with a per device state which is initialised
> > and allocated during probing.
> >
> > Signed-off-by: Ojaswin Mujoo <ojaswin98@xxxxxxxxx>
> ...
> >
> > /*
> > @@ -1763,6 +1795,7 @@ static int vchiq_probe(struct platform_device *pdev)
> > struct device_node *fw_node;
> > const struct of_device_id *of_id;
> > struct vchiq_drvdata *drvdata;
> > + struct vchiq_device *vchiq_dev;
> > int err;
> >
> > of_id = of_match_node(vchiq_of_match, pdev->dev.of_node);
> > @@ -1784,7 +1817,18 @@ static int vchiq_probe(struct platform_device *pdev)
> >
> > platform_set_drvdata(pdev, drvdata);
> >
> > - err = vchiq_platform_init(pdev, &g_state);
> > + vchiq_dev = kzalloc(sizeof(struct vchiq_device), GFP_KERNEL);
> > + vchiq_dev->state = kzalloc(sizeof(struct vchiq_state), GFP_KERNEL);
> > + vchiq_dev->vchiq_pdev = *pdev;
> > +
> > + g_state = vchiq_dev->state;
> > +
>
> just a quick idea: how about storing the global state within vchiq_drvdata?
>
> So there is no need to reinvent somekind of vchiq device which is the
> "same" as the platform device. After that you are able to access the
> private driver data via platform_get_drvdata().
Hi Stefan,

Thank for looking into this. I agree that the vchiq_device is just a
sorta extension of the pdev. However, regarding using the drvdata, I had
some questions.

So I understand the purpose of this patch is to make sure our driver is
able to support multiple devices, that is it can work with say, an SoC
with 2 VideoCore VPUs. In this case, we would need to maintain 2 states
for each VPU instead of a global state, however if we use something like
the following:

drvdata = (struct vchiq_drvdata *)of_id->data;
drvdata->state = kzalloc(sizeof(struct vchiq_state), GFP_KERNEL);
platform_set_drvdata(pdev, drvdata);

Correct me if I'm wrong but I think the assignment to drvdata in line 1
above will return a pointer to the same structure. In which case, the
line 2 will always overwrite the older state that is present.
That's why I was initialising a separate object (vchiq_device) everytime
the probe was called so that each device can have their own device
specific structs.

Please let me know if my understanding of drvdata is wrong here.
>
> Best regards
>

Thank you!
Ojaswin