linux-kernel - Re: [PATCH v10 2/3] tty: hvc: pass DMA capable memory to put

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Date:   Sat, 9 Oct 2021 13:58:43 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Xianting Tian <xianting.tian@...ux.alibaba.com>
Cc:     jirislaby@...nel.org, amit@...nel.org, arnd@...db.de,
        osandov@...com, shile.zhang@...ux.alibaba.com,
        linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 2/3] tty: hvc: pass DMA capable memory to put_chars()

On Sat, Oct 09, 2021 at 07:48:28PM +0800, Xianting Tian wrote:
> --- a/drivers/tty/hvc/hvc_console.h
> +++ b/drivers/tty/hvc/hvc_console.h
> @@ -32,13 +32,21 @@
>   */
>  #define HVC_ALLOC_TTY_ADAPTERS	8
>  
> +/*
> + * These sizes are most efficient for vio, because they are the
> + * native transfer size. We could make them selectable in the
> + * future to better deal with backends that want other buffer sizes.
> + */
> +#define N_OUTBUF	16
> +#define N_INBUF		16
> +
> +#define __ALIGNED__ __attribute__((__aligned__(sizeof(long))))

Does this conflict with what is in hvcs.c?

> +
>  struct hvc_struct {
>  	struct tty_port port;
>  	spinlock_t lock;
>  	int index;
>  	int do_wakeup;
> -	char *outbuf;
> -	int outbuf_size;
>  	int n_outbuf;
>  	uint32_t vtermno;
>  	const struct hv_ops *ops;
> @@ -48,6 +56,18 @@ struct hvc_struct {
>  	struct work_struct tty_resize;
>  	struct list_head next;
>  	unsigned long flags;
> +
> +	/* the buf is used in hvc console api for putting chars */
> +	char cons_outbuf[N_OUTBUF] __ALIGNED__;
> +	spinlock_t cons_outbuf_lock;

Did you look at the placement using pahole as to how this structure now
looks?

> +
> +	/* the buf is for putting single char to tty */
> +	char outchar;
> +	spinlock_t outchar_lock;

So you have a lock for a character and a different one for a longer
string?  Why can they not just use the same lock?  Why are 2 needed at
all, can't you just use the first character of cons_outbuf[] instead?
Surely you do not have 2 sends happening at the same time, right?

> +
> +	/* the buf is for putting chars to tty */
> +	int outbuf_size;
> +	char outbuf[0] __ALIGNED__;

I thought we were not allowing [0] anymore in kernel structures?

thanks,

greg k-h