Re: [PATCH v4 1/4] usb: dbc: early driver for xhci debug capability

From: Thomas Gleixner
Date: Wed Nov 09 2016 - 04:25:55 EST


On Tue, 1 Nov 2016, Lu Baolu wrote:
> +static int __init xdbc_init(void)
> +{
...
> + base = ioremap_nocache(xdbc.xhci_start, xdbc.xhci_length);
> + if (!base) {
> + xdbc_trace("failed to remap the io address\n");
> + ret = -ENOMEM;
> + goto free_and_quit;
> + }
> +
> + early_iounmap(xdbc.xhci_base, xdbc.xhci_length);
> + xdbc_trace("early mapped IO address released\n");
> +
> + xdbc.xhci_base = base;
> + offset = xhci_find_next_ext_cap(xdbc.xhci_base, 0, XHCI_EXT_CAPS_DEBUG);
> + xdbc.xdbc_reg = (struct xdbc_regs __iomem *)(xdbc.xhci_base + offset);

This is broken. What prevents that

- a printk is in progress on another cpu?

- a printk happens between the unmap and storing the new base ?

Nothing AFAICT. So this needs to be done in a safe way. And just making it

oldbase = xdbc.xhci_base;
base = ioremap(....);
xdbc.xhci_base = base;
early_iounmap(oldbase);

does not work either because the compiler can rightfully cache
xdbc.xhci_base in the write related functions. The same issue with
xdbc.xdbc_reg.

Thanks,

tglx