linux-kernel - Re: [PATCH] tty: hvc: don't allocate a buffer for console print on stack

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Date:   Fri, 17 Mar 2017 14:09:30 +0900
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Jan Dakinevich <jan.dakinevich@...il.com>
Cc:     Jiri Slaby <jslaby@...e.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Arnd Bergmann <arnd@...db.de>, Amit Shah <amit@...nel.org>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: hvc: don't allocate a buffer for console print on
 stack

On Fri, Feb 17, 2017 at 11:42:45PM +0300, Jan Dakinevich wrote:
> The buffer is used by virtio console driver as DMA buffer. Since v4.9
> (if VMAP_STACK is enabled) we shouldn't use the stack for DMA.

You shouldn't use 'static' data either, that's not always guaranteed to
be DMA-able, right?

> 
> Signed-off-by: Jan Dakinevich <jan.dakinevich@...il.com>
> ---
>  drivers/tty/hvc/hvc_console.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
> index 9b5c0fb..1ce6aaf 100644
> --- a/drivers/tty/hvc/hvc_console.c
> +++ b/drivers/tty/hvc/hvc_console.c
> @@ -143,10 +143,15 @@ static struct hvc_struct *hvc_get_by_index(int index)
>  static void hvc_console_print(struct console *co, const char *b,
>  			      unsigned count)
>  {
> -	char c[N_OUTBUF] __ALIGNED__;
>  	unsigned i = 0, n = 0;
>  	int r, donecr = 0, index = co->index;
>  
> +	/*
> +	 * Access to the buffer is serialized by console_sem in caller code from
> +	 * kernel/printk/printk.c
> +	 */
> +	static char c[N_OUTBUF] __ALIGNED__;

What about allocating it dynamically?  That's the correct thing to do.

thanks,

greg k-h