linux-kernel - Re: earlyprintk=xdbc seems broken

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Date:   Mon, 20 Dec 2021 16:06:50 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        Chunfeng Yun <chunfeng.yun@...iatek.com>
Subject: Re: earlyprintk=xdbc seems broken

On Mon, Dec 20, 2021 at 03:40:17PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 17, 2021 at 04:19:20PM +0100, Greg KH wrote:
> > On Fri, Dec 17, 2021 at 02:55:07PM +0100, Peter Zijlstra wrote:
> > > On Fri, Dec 17, 2021 at 01:01:43PM +0200, Mathias Nyman wrote:
> > > > I can reproduce this.
> > > > Looks like problems started when driver converted to readl_poll_timeout_atomic() in:
> > > > 
> > > > 796eed4b2342 usb: early: convert to readl_poll_timeout_atomic()
> > > 
> > > I can confirm, reverting that solves the boot hang, things aren't quite
> > > working for me though.
> > > 
> > > > Seems to hang when read_poll_timeout_atomic() calls ktime_* functions.
> > > > Maybe  it's too early for ktime.
> > > 
> > > It certainly is, using ktime for delay loops sounds daft to me anyhow.
> > 
> > It was a "find a pattern and replace it with a function call" type of
> > cleanup series.  It's obviously wrong, I will go revert it now.
> 
> 796eed4b2342 is definitely wrong, but instead of a straight up revert,
> perhaps do something like this ?
> 
> ---
> Subject: usb: early: Revert from readl_poll_timeout_atomic()
> 
> Reverts commit 796eed4b2342 ("usb: early: convert to
> readl_poll_timeout_atomic()") and puts in a comment to avoid the same
> happening again.
> 
> Specifically that commit is wrong because readl_poll_timeout_atomic()
> relies on ktime() working while this earlyprintk driver should be usable
> long before that.
> 
> Fixes: 796eed4b2342 ("usb: early: convert to readl_poll_timeout_atomic()")
> Debugged-by: Mathias Nyman <mathias.nyman@...ux.intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radaed.org>
> ---
>  drivers/usb/early/xhci-dbc.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/early/xhci-dbc.c b/drivers/usb/early/xhci-dbc.c
> index 933d77ad0a64..ff279a830653 100644
> --- a/drivers/usb/early/xhci-dbc.c
> +++ b/drivers/usb/early/xhci-dbc.c
> @@ -14,7 +14,6 @@
>  #include <linux/pci_ids.h>
>  #include <linux/memblock.h>
>  #include <linux/io.h>
> -#include <linux/iopoll.h>
>  #include <asm/pci-direct.h>
>  #include <asm/fixmap.h>
>  #include <linux/bcd.h>
> @@ -136,9 +135,20 @@ static int handshake(void __iomem *ptr, u32 mask, u32 done, int wait, int delay)
>  {
>  	u32 result;
>  
> -	return readl_poll_timeout_atomic(ptr, result,
> -					 ((result & mask) == done),
> -					 delay, wait);
> +	/*
> +	 * This must not be readl_poll_timeout_atomic(), as this is used
> +	 * *early*, before ktime lives.
> +	 */
> +	do {
> +		result = readl(ptr);
> +		result &= mask;
> +		if (result == done)
> +			return 0;
> +		udelay(delay);
> +		wait -= delay;
> +	} while (wait > 0);
> +
> +	return -ETIMEDOUT;
>  }
>  
>  static void __init xdbc_bios_handoff(void)

Please see c4d936efa46d ("Revert "usb: early: convert to
readl_poll_timeout_atomic()"") in Linus's tree.  I already added a
comment much like yours.  If that's not sufficient, I'll be glad to
re-word it.

thanks,

greg k-h