Re: [PATCH v6 2/2] soc: mediatek: add mt6779 devapc driver

[Neal Liu <neal.liu@mediatek.com>]

Hi Chun-Kuang,

On Sat, 2020-08-15 at 11:03 +0800, Chun-Kuang Hu wrote:
> Hi, Neal:
> 
> Neal Liu <neal.liu@mediatek.com> 於 2020年8月13日 週四 上午11:33寫道：
> >
> > MediaTek bus fabric provides TrustZone security support and data
> > protection to prevent slaves from being accessed by unexpected
> > masters.
> > The security violation is logged and sent to the processor for
> > further analysis or countermeasures.
> >
> > Any occurrence of security violation would raise an interrupt, and
> > it will be handled by mtk-devapc driver. The violation
> > information is printed in order to find the murderer.
> >
> > Signed-off-by: Neal Liu <neal.liu@mediatek.com>
> > ---
> 
> [snip]
> 
> > +/*
> > + * devapc_violation_irq - the devapc Interrupt Service Routine (ISR) will dump
> > + *                        violation information including which master violates
> > + *                        access slave.
> > + */
> > +static irqreturn_t devapc_violation_irq(int irq_number,
> > +                                       struct mtk_devapc_context *ctx)
> > +{
> > +       /*
> > +        * Mask slave's irq before clearing vio status.
> > +        * Must do it to avoid nested interrupt and prevent
> > +        * unexpected behavior.
> > +        */
> > +       mask_module_irq(ctx, true);
> 
> I still don't understand why nested interrupt happen. If two CPU
> process different devapc interrupt at the same time, mask interrupt
> could not prevent these two CPU to sync vio dbg at the same time. As I
> know, in ARM CPU, only CPU0 process irq handler, and all devapc
> interrupt has the same priority, so why nested interrupt happen? Could
> you explain more detail about how nested interrupt happen?

If there is another violation happened before previous violation is
fully handled, nested interrupt would happen.

Let's me take an example:
vio A happen
enter A ISR
...		vio B happen
finish A ISR	enter B ISR
		...
		finish B ISR

We mask all module's irq to avoid nested interrupt.

> 
> > +
> > +       while (devapc_sync_vio_dbg(ctx))
> > +               devapc_extract_vio_dbg(ctx);
> > +
> > +       /*
> > +        * Ensure that violation info are written
> > +        * before further operations
> > +        */
> > +       smp_mb();
> > +
> > +       clear_vio_status(ctx);
> > +       mask_module_irq(ctx, false);
> > +
> > +       return IRQ_HANDLED;
> > +}
> > +
> 
> [snip]
> 
> > +
> > +static int mtk_devapc_remove(struct platform_device *pdev)
> > +{
> > +       struct mtk_devapc_context *ctx = platform_get_drvdata(pdev);
> > +
> > +       stop_devapc(ctx);
> > +
> > +       if (ctx->infra_clk)
> 
> This always true.

Does it mean that remove function would be called only if probe function
is returned successfully?
Is there any chance this function would be called directly?

> 
> Regards,
> Chun-Kuang.
> 
> > +               clk_disable_unprepare(ctx->infra_clk);
> > +
> > +       return 0;
> > +}
> > +
> > +static struct platform_driver mtk_devapc_driver = {
> > +       .probe = mtk_devapc_probe,
> > +       .remove = mtk_devapc_remove,
> > +       .driver = {
> > +               .name = KBUILD_MODNAME,
> > +               .of_match_table = mtk_devapc_dt_match,
> > +       },
> > +};
> > +
> > +module_platform_driver(mtk_devapc_driver);
> > +
> > +MODULE_DESCRIPTION("Mediatek Device APC Driver");
> > +MODULE_AUTHOR("Neal Liu <neal.liu@mediatek.com>");
> > +MODULE_LICENSE("GPL");
> > --
> > 1.7.9.5
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